I've fitted one of those brass thread filling valves to my cistern. Don't get on with plastic when the pipes are copper.
Tightening up the tap fitting is presenting a problem. There is very little brass thread projecting from the cistern - although it does have flats on it. Which measure at 18mm. So needs a very slim spanner to go between the tap fitting nut and the one holding the valve into the cistern. Needs to be less than 4mm thick.
You can buy slim spanners at a price. But this doesn't need to be as strong as you'd need for a nut that size on a car, etc. One of the old pressed steel bike types would probably be strong enough.
The new cistern is a very expensive Ideal Standard one. It's a high level type with a chain for the flush. The lever simply didn't have enough range to operate the syphon. Pull the wire link on the syphon by hand and it worked just fine. Went as far as phoning their help line. No help there - other than take pics and send to them.
After a deal of head scratching - the lever looked very well made and polished chrome - it turned out it had been fitted to the bracket the wrong way round. Pressed out the roll pin pivot and re-assembled and it now works.