Simon Avery wrote:

However... When I hold down the ballcock on mine to simulate an
overflow, water DOES drain down to the pan (as per the new regs that
allow an internal overflow). I have absolutely no idea how it does
this - there's no physical difference to the assembly compared to a
normal one, but it "just works", so I stopped worrying about it. It
doesn't drain enough to cope with a full blown entry failure, but then
- a 21.5mm conventional overflow often wouldn't either.

On reading this thread I decided to check 2 cisterns I've installed in my
house recently (different cisterns but both with internal overflows). I
held the ballcock right down on both and both coped totally - the water
level rose slightly but then remained constant - overflow going down the
pan. They would certainly cope with a full blown entry failure.
--
Regards,

Tròy the Black Lab.