Neil Jones wrote:

Depends if you need to reuse the damaged screw - if not maybe put a small
blob of araldite or some other adhesive on the end of the allen key, insert
into the stripped socket and wiat for a bond to form, then you can remove
the screw.

Neil

I probably would short term - i.e. to get the shower working again. In
the meantime I'd probably give the shower company another call, as I've
not seen a similar type of screw being sold.

I did think of that... and it might come to it if all else fails. Ta!