From the previous thread you started: this is your DPs pension, and you have an accountant doing the tax return?
I'm trying to read between the lines since there are variations of defined contribution pensions. Is the 20% a UK Government contribution to the pension, as well as DPs contributions and the employers contributions? Or, is the 20% the amount of tax you think HMRC didn't tax DPs contributions because he is (?) in the 20% tax band?
Your accountant will be able to decipher DPs P60, which will be issued after 5 April, as to gross income, taxed income, untaxed income (including DPs pension contributions?), and amount of tax withheld on taxed income.
If DP is resident in the UK on April 15, he will have until June 15 to file the return, or the accountant may request a longer extension.
Make sure your accountant understands how the use of treaty benefits, if exercised, eventually impact on the taxation of the pension (and depending on residence in either country) once DP is drawing the pension.