This latter situation may well be the problem. I've no way of knowing
whether the base of the channel where it joins the wall was properly
sealed , and I think I will have to resort to removing it from the wall
to check.

With regard to allowing a gap to let water run out, the shower was
originally installed with sealant around the outside of the base of the
enclosure. I'd assumed this was to let any water run back into the
shower tray, but in fact it seems that this is a good way for it to get
into the frame. It runs down the glass, finds the gap then seems to go
in. I filled it both sides but this didn't help however and I've taken
the inner sealant off again.