In message , at 11:21:43 on Thu, 15 Oct
2020, tim... remarked:
Or get a Eurolock cylinder that works even with the key in the other
side...

I see I was unclear in my upstream post: Cylinders can be bought
either way. The more expensive ones can be changed over, though this
may require knowledge and/or tools to contain tiny springybits,
and/or small precision bits to exchange. (Disassembly tends to be
much easier than assembly!)

Won't DIY work of that kind invalidate the kitemark, and thus your
home insurance?

changing a like for like type cylinder wont

putting in a less secure cylinder might

I don't know what kind of certification a "locksmith" would need, to
maintain the kitemark status.

changing a cylinder in an open unlocked door is easily DIY-able.

It requires almost zero skills. Just knowledge of the "trick"

None of those sounds like "disassembly", with tiny springy bits.
--
Roland Perry