How do I fit restrictor in toilet pan inlet when plumbing? Water is overflowing the pan and soaking the floor each time flush is pulled. Armitage advised to fit this restrictor.
Actually, let's start with some disambiguation, as we may be separated by a common language. I've been assuming you mean the lower portion of the toilet when you say "pan" - is that correct?
Given low detail and no picture in the question, I'll go on first principles and guess that a "toilet pan inlet restrictor" would involve removing the cistern/tank from the pan/bowl (generally a couple of screws or bolts, plus the plumbing) inserting the restrictor, and reinstalling the tank/cistern on the bowl/pan. And I'll second @bcworkz's comment that you should really make sure the outlet is not restricted - an anemic (restricted) flush into a bowl/pan with a restricted outlet is a recipe for a lot of clogs and flushes that fail to flush.
An easier version might install by simply lifting the flapper/flush valve and dropping it in place below the valve. But you may have an "outlet siphon" instead of a flapper valve, since your terminology appears to be British. That most likely does involve removing the cistern.