What thread is on the bottom of a toilet fill valve?

Probably BSP (British Standard Pipe) The rigid pipe is best as it makes the whole assembly stiffer, especially where you have the twisting/leverage effects of a float valve.

It's pretty certain to be 1/2"BSP (which has an OD of approx 3/4" because the 1/2" refers to the nominal bore).

If the joint is leaking, it's likely that the fibre washer has perished. You can get a new one for pence which should cure it.

UNLESS you have the problem which I had recently - where the end of the bottom-entry plastic float valve had started to disintegrate - so I had to replace the whole valve.

Why not use a flexible tap connector?

Thanks.

The other end is a 15mm isolating valve and ideally I'd like to connect to the valve rather than put in a short run of copper pipe.

I think I want a flexi hose that 1/2 BSP to what ever thread is on the

The weight tests and claims made on these enrollments is build only in ligh t of my own experiences with the ones showed in the video. Solitary results may change, and caution and care should be taken while stacking the valves under high weight. The risk of higher weights is that the balls may be com pelled from the connector, shooting out like shots. High weights may simila rly influence the ball to rush up, turning away run of the mill action of t he valve, or possibly even fundamental disillusionment of the valve all thi ngs considered. These valves are not made, or affirmed, to be used as a pie ce of any stone strong exercises.

My hovercraft is full of eels.

Yes, I hope he got a good crop of Leaks in 2013. Brian