Yearly pension contribution allowance is £40,000, but you can contribute more than that if you haven't used the full allowance from any of the 3 previous tax years. I get that bit.
If you want to use this carry forward allowance am I right in thinking I don't need to do anything official, just make sure I don't use too much and prepare to justify if HMRC come asking? (I do not do a tax return BTW).