What's the average FTSE level?

You would need to add up all daily closure figures since your required date, and divide by the number of days the market was open for, in order to obtain an average closing figure!

You don't need to work out the average anyway, a gauranteed equity bond simply tracks growth for the time period you took it out until the maturity date. If it has reached a higher level then you get a payout according to its growth. If it hasn;t grown you only get your initial deposit back! The Ftse 100 is actually lower now than when you put it out at 4229.73 as of yesterday. So unless it grows to above 4374.5 by 24 Nov 2008 you wont get anything more than your deposit back

Ok you've been slightly unlucky with the timing but I think a tracker must rate over this bond.
I thought some of these bonds gave 25% as a minimum

The returns on NS&I Guaranteed Equity Bonds are linked to the performance of the FTSE 100 index over a set term. If the FTSE 100 index grows over the term, you will benefit from that growth. And if it falls, you will still get back 100% of your original investment – guaranteed.
Please note that the FTSE 100 index measures share prices only, so you will not benefit from dividends. This means that you may not get as high a return as you would with a direct stock market investment. The returns on our Guaranteed Equity Bonds are subject to averaging and are liable to UK Income Tax on maturity. Although your capital is fully guaranteed, inflation may reduce the true value of your money over the investment term.

http://www.nsandi.com/products/geb/index.jsp


They will measure closing day ftse figures and average that like you say.

Date,Open Price,Close Price,High Price,Low Price,Volume
27-May-2008,6117.30,6058.50,6141.70,6048.70,0
28-May-2008,6068.10,6069.60,6122.20,6052.50,0
29-May-2008,6091.00,6068.10,6130.50,6041.10,0
30-May-2008,6074.10,6053.50,6111.60,6044.70,0
02-Jun-2008,6059.00,6007.60,6060.80,5978.40,0
03-Jun-2008,5997.20,6057.70,6059.00,5993.30,0
04-Jun-2008,6024.90,5970.10,6030.20,5933.30,0
05-Jun-2008,5969.60,5995.30,6005.10,5941.00,0
06-Jun-2008,6020.80,5906.80,6074.50,5906.50,0
09-Jun-2008,5902.90,5877.60,5938.50,5869.60,0
10-Jun-2008,5869.80,5827.30,5873.20,5813.40,0
11-Jun-2008,5847.40,5723.30,5859.40,5708.30,0
12-Jun-2008,5725.50,5790.50,5797.80,5719.80,0
13-Jun-2008,5793.70,5802.80,5818.30,5719.50,0
16-Jun-2008,5817.20,5794.60,5832.60,5759.60,0
17-Jun-2008,5799.70,5861.90,5930.30,5799.70,0
18-Jun-2008,5846.80,5756.90,5846.80,5735.40,0
19-Jun-2008,5752.20,5708.40,5786.40,5707.20,0
20-Jun-2008,5708.80,5620.80,5731.90,5597.00,0
23-Jun-2008,5623.50,5667.20,5679.20,5603.20,0
24-Jun-2008,5671.70,5634.70,5693.80,5581.60,0
25-Jun-2008,5644.30,5666.10,5669.60,5636.50,0
26-Jun-2008,5648.40,5518.20,5648.40,5518.20,0
27-Jun-2008,5507.00,5529.90,5555.20,5470.90,0
30-Jun-2008,5527.00,5625.90,5625.90,5520.20,0
01-Jul-2008,5600.20,5479.90,5601.50,5466.30,0
02-Jul-2008,5480.70,5426.30,5566.80,5426.30,0
03-Jul-2008,5400.00,5476.60,5490.60,5358.50,0
04-Jul-2008,5464.40,5412.80,5490.20,5394.00,0
07-Jul-2008,5439.00,5512.70,5516.40,5400.10,0
08-Jul-2008,5446.40,5440.50,5487.60,5358.70,0
09-Jul-2008,5465.10,5529.60,5538.20,5465.10,0
10-Jul-2008,5498.10,5406.80,5498.10,5392.60,0
11-Jul-2008,5420.50,5261.60,5461.80,5261.60,0
14-Jul-2008,5304.00,5300.40,5373.20,5293.00,0
15-Jul-2008,5279.70,5171.90,5279.70,5119.00,0
16-Jul-2008,5178.60,5150.60,5210.10,5071.10,0
17-Jul-2008,5176.60,5286.30,5320.50,5176.60,0
18-Jul-2008,5254.80,5376.40,5376.40,5216.60,0
21-Jul-2008,5375.70,5404.30,5445.80,5334.60,0
22-Jul-2008,5383.90,5364.10,5383.90,5282.80,0
23-Jul-2008,5404.40,5449.90,5467.20,5381.90,0
24-Jul-2008,5458.70,5362.30,5463.70,5345.10,0
25-Jul-2008,5336.30,5352.60,5375.20,5291.40,0
28-Jul-2008,5355.20,5312.60,5365.80,5308.40,0
29-Jul-2008,5297.80,5319.20,5354.60,5261.40,0
30-Jul-2008,5346.00,5420.70,5435.90,5346.00,0
31-Jul-2008,5440.70,5411.90,5456.10,5371.30,0
01-Aug-2008,5398.30,5354.70,5409.30,5321.30,0
04-Aug-2008,5351.40,5320.20,5414.70,5310.30,0
05-Aug-2008,5318.80,5454.50,5454.50,5299.70,0
06-Aug-2008,5470.50,5486.10,5498.60,5440.10,0
07-Aug-2008,5480.60,5477.50,5539.30,5450.90,0
08-Aug-2008,5456.60,5489.20,5507.20,5410.80,0
11-Aug-2008,5494.90,5541.80,5541.80,5477.70,0
12-Aug-2008,5528.80,5534.50,5569.20,5491.30,0
13-Aug-2008,5517.10,5448.60,5517.80,5437.10,0
14-Aug-2008,5524.50,5497.40,5539.60,5445.60,0
15-Aug-2008,5508.10,5454.80,5538.80,5432.20,0
18-Aug-2008,5449.20,5450.20,5498.70,5425.30,0
19-Aug-2008,5417.10,5320.40,5417.10,5317.10,0
20-Aug-2008,5338.60,5371.80,5384.20,5325.80,0
21-Aug-2008,5360.10,5370.20,5408.00,5311.40,0
22-Aug-2008,5371.10,5505.60,5505.60,5368.00,0
26-Aug-2008,5463.10,5470.70,5493.00,5369.30,0
27-Aug-2008,5469.80,5528.10,5540.80,5434.70,0
28-Aug-2008,5522.10,5601.20,5634.00,5496.90,0
29-Aug-2008,5615.40,5636.60,5649.10,5587.40,0
01-Sep-2008,5621.50,5602.80,5621.50,5573.80,0
02-Sep-2008,5599.90,5620.70,5646.50,5574.90,0
03-Sep-2008,5593.80,5499.70,5593.80,5492.10,0
04-Sep-2008,5503.20,5362.10,5541.70,5362.10,0
05-Sep-2008,5349.80,5240.70,5359.30,5227.60,0
08-Sep-2008,5304.40,5446.30,5447.60,5304.40,0
09-Sep-2008,5455.20,5415.60,5524.80,5386.90,0
10-Sep-2008,5396.30,5366.20,5419.50,5327.80,0
11-Sep-2008,5362.10,5318.40,5377.90,5258.10,0
12-Sep-2008,5347.30,5416.70,5416.70,5320.70,0
15-Sep-2008,5377.80,5204.20,5377.80,5124.90,0
16-Sep-2008,5149.20,5025.60,5149.20,4961.20,0
17-Sep-2008,5038.00,4912.40,5124.40,4903.30,0
18-Sep-2008,4920.60,4880.00,5015.90,4860.70,0
19-Sep-2008,4884.60,5311.30,5351.20,4857.10,0
22-Sep-2008,5311.33,5236.26,5339.25,5236.26,0
23-Sep-2008,5232.09,5136.12,5232.09,5076.32,0
24-Sep-2008,5149.17,5095.57,5167.36,5087.30,0
25-Sep-2008,5088.86,5197.02,5211.64,5060.74,0
26-Sep-2008,5166.89,5088.47,5166.89,5057.33,0
29-Sep-2008,5063.32,4818.77,5063.32,4818.77,0
30-Sep-2008,4784.53,4902.45,4953.40,4671.02,0
01-Oct-2008,4941.49,4959.59,5012.24,4891.46,0
02-Oct-2008,4963.47,4870.34,5052.00,4862.11,0
03-Oct-2008,4866.98,4980.25,5003.90,4832.10,0
06-Oct-2008,4944.71,4589.19,4944.71,4549.66,0
07-Oct-2008,4606.72,4605.22,4745.01,4517.47,0
08-Oct-2008,4529.64,4366.69,4654.18,4245.29,0
09-Oct-2008,4399.56,4313.80,4512.45,4274.40,0
10-Oct-2008,4311.03,3932.06,4311.03,3873.99,0
13-Oct-2008,3943.97,4256.90,4256.90,3943.97,0
14-Oct-2008,4301.53,4394.21,4534.35,4256.90,0
15-Oct-2008,4385.79,4079.59,4394.21,4051.98,0
16-Oct-2008,4043.01,3861.39,4063.56,3808.11,0
17-Oct-2008,3909.55,4063.01,4077.58,3861.29,0
20-Oct-2008,4095.17,4282.67,4282.67,4069.71,0
21-Oct-2008,4310.44,4229.73,4347.69,4212.37,0

http://www.digitallook.com

Excel will take this afaik, its csv and you could average it out or if not I'll do it later to give you some idea of your current return.
Hopefully you will be lucky and the market will just stay even till your close of averaging, to simulate this you add in the remaining days with the last figure



edit:
May 24th - Nov 22nd average ftse was 5315.7

If the ftse was 4040 till 24th of Nov then the overall average would be 5096.6 which I think gives the bond a return of 16.5% or something like 3.3% a year