GMM wrote:

What amazes me, though, is why at least some of the seals and bits
aren't stocked by the sheds. Surely there must be plenty of demand.

When I first started playing DIY as a teenager in the late 80s, when B&Q
was the same as Homebase, etc, I could never get my head round that they
never seemed to have a cohesive range of, say, plumbing bits. In time a)
B&Q got better and b) I realised that the stock wasn't what was needed
so much as "what people (randomly) bought" and therefore what got
re-ordered.

--
Scott