Hi,
Anytime someone has a shower upstairs, the water is dripping from the ceiling downstairs. We have double-checked everything, and it is the sealant where the shower tray meets the wall. If we remove the old sealing and re-seal it, there is no leaking for a couple of months but after a while (from the slight movement of the tray) the sealing cracks and it`s dripping again.
We could play the re-sealing game every 2-3 months but is there a better solution? Maybe a special type of moulding? We don`t really want to rip the shower tray and the tiles out.
Thanks!

Trays aren't supposed to move, they do deflect down from your weight but not side to side. Unless you can stabilize the pan, installed poorly, you will keep playing the sealing game.

Bite the bullet.
You need a new fully sealed copper tray.
The damage being caused to the building
far exceeds the replacement value.

Replacement does sound like an option but a copper pan is beyond most DIYer, I don't even do them since there are many cheaper, easier and better options.