I'm in Midlands so a fair way from you I'm afraid.
If the tiles are good and not blown or falling off the walls and leak is from the corner where shower screen is (which is common) then I wouldn't be taking the tray out.
I would remove the screen and the sealant all round the tray, clean and thoroughly dry.
Either clean up old screen with razor blade or replace with new. You will clearly see with screen off if it was incorrectly sealed as there will be a gap in the seal where the screen profile was.
Then its just a case of sealing around the (clean and dry) tray before refitting the screen, it won't hurt to fit the screen a day after sealing the tray but I often do same day (with caution . Most screens ask for sealant on the OUTSIDE ONLY so that any water that gathers in the profile can drain freely back into the tray.
This is a fairly common fault that I've seen many bathroom fitters make even (and especially) on newbuild sites. It's a chunk of work for what is a simple fix and I usually charge a days labour.
I've tried other 'fixes' over the years like drilling and forcing silicone into the gap but find the above is the only acceptable option for me.
Of course this may not be your problem but as taking the screen out is going to be part of any further work then it won't hurt to take it out and have a look?