Thanks for all your help with the water butts. Just to finish things
off:

I've followed the advice I have received here and rather than use a
diverter, I have connected the drain pipe to go into the butt. There
is even a template on the lid that says "cut here for pipe".

Does it matter how deep the pipe goes into the butt? One post
mentioned taking the pipe almost to the bottom but with a small gap to
allow for any sediment. Is this the best solution? is there a reason
that the pipe has to be submerged? Why not just have a short pipe that
finishes nearer the top of the butt? This would disturb the surface of
the water when it rains; I don't know whether that makes any
difference?

People have mentioned being able to dismantle it for cleaning out
every couple of years. I think I need to solvent weld the elbows onto
the down pipe otherwise they will fall off. I made the mistake of
spraying them with silicone spray so that I could insert them easily
and now they keep falling off! Should I leave one un-glued so that I
can pull it off for servicing?

I thought the lid needed to be turned to remove it, which would be a
problem with a pipe going through it, however, it looks as though
there are just four clips. If these were all pressed in, I imagine it
could be lifted straight off.

Thanks.