Screwed on Cash ISA interest rate

I have a cash ISA with Santander. I opened it in 2006 with Abbey as a Postal ISA, however, a couple of years ago I was advised by the bank to change to a Direct ISA. I was happy with the rate which was always competitive with the market. I left around £38,000 in the account throughout 2009 -2010 but I have just discovered that the bank decided to reduce the rate on this product to 0.5% at the start of this last year for customers who had had the account over 2 years; consequently when I was expecting to receive over £800 I have ended up with under £150. The bank did not inform me of this or of course I would have changed to a different provider. When I spoke to them they told me that I had 'Issue 1' of the Direct ISA but it had now been updated to 'Issue 6'; this information is not apparent in the marketing of Santander and when I told them about it they have offered to upgrade me to the normal Direct ISA. Of course this does not help me with the money I have lost for this tax year.
This seems like a scam to deprive people of their interest. It is not fair that they can reduce the interest rate on this product without informing me - they claim a letter was sent out but I never received it. What can I do about this? Is there any way I can contest this or have I been screwed?
Can anyone advise on this...Duncan