The history is that when I noticed an overflow from the cistern and went to fix it I discovered the flush pipe connection was leaking at the cistern end. It's taken a while but, with some useful tips from here, the flush pipe is now water tight again.
So to the float valve - no chance of repairing the original so needs replacing. The brass replacement I have has no "silencing mechanism". The water issues into the cistern from a 12mm diameter hole in the brass body. Any suggestion please for how to fit some sort of tubing to silence the flow?
I see that modern variants of the float valve have no long arm and float but the float rides up a vertical guide. None of the on-line sources I can find give the dimensions of any of these attractive alternatives. I don't think there is any possibility that they are small enough to fit in the cistern "round" the siphon base. So I might swap over the float valve and the overflow inputs to the cistern where there should be no problem with space for the float valve. Is it worth the effort?
Apologies for the length Frank