What does HMRC do with the other 25% on pension contribution?

BoGoF

TheAble said:

Out of interest if you're a 40% taxpayer what number do you put in your tax return if you top up your pension from your bank account? Say I put in £100, do I declare £125?

Also, suppose my taxable income from my job is £100,125. Is £100 the correct amount to top up my pension by if I want to get down to £100k, for the purposes of keeping my personal allowance intact for instance?

(Sorry if I'm hijacking the thread but felt a relevant continuation. Many thanks.)

What does the Tax Return tell you to put. I'll save you looking - the gross amount.

For your 2nd point, yes assuming you have no benefits in kind or investment income.