Cistern flush handle fittings

[grumpyoldjoe]

Trying to fit a new flush handle etc to my cistern.
The hole in the cistern looks to be 20mm but I suspect its slightly less than this and probs 3/4 inch in old money.
I have tried 3 different fittings from local B+Q and have managed to ruin 2 as the hole is just to tight. Searched on net but dont see smaller kit.
Can someone point me in the direction to a supplier?
Thanks in advance
Joe