Are they all a standard fitting? i.e. if i can’t find an exact replacement will any flush valve do the job?

yes any one will do

But if the syphon is blue plastic you just pull a pin out to remove it and simply pop a new one on

I’ll take a photo. Stupid thing’s sticking and just trickling water all the time

That happens here with just a little lime scale build up on the pin and diaphragm.

Having taken the valve out it appears the lever that lifts the valve has slightly chipped, and it’s such a delicate design that as a result, it now misses the guide and so gets stuck at the bottom of it’s action.

New valve needed i think.

No major problem though – luckily the valve from the downstairs loo fits and is working so we’re ok.

Thanks for your help chaps.