How to calculate a tax year's pension

jimmo

If you want to be strictly accurate pensions are assessable on the accruals (entitlement) basis, not what has been received.
http://www.hmrc.gov.uk/manuals/eimanual/EIM74101.htm
In the original question the amount assessable for 2005/06 would be;
6/4/05 to 10/4/05, 5/7 * a unknown amount.
11/4/05 to 3/4/06 51 weeks @ 99.87 = £5093.37
4/4/06 to 5/4/06 2/7*99.78 = £28.53.
Does it matter?
Chances are that HMRC coding and assessing tolerances mean that the taxpayer, the OP's father, has underpaid a £ or two each year and HMRC are happy to live with that.
It is, of course possible that there is an overpayment but, unless there is something obvious then someone who struggles with basic maths will really struggle with HMRC.