two thoughts
1. check rules on diy repairs (UK) water & electric
2. have you tried www. spares4u.or other sites they might have pattern parts.
reason discontinued might be a part not held for long as not called for
Old shower unit.
No manufacturer is obligated to stock / supply spare parts. Only buy "pattern part"( same part made by another manufacturer) from reputable outlets, to many dubious item on market not fit for purpose.
Bobuk
March 2021