Advice needed on consumer rights with particular reference to opticians

Two weeks ago, I had an eye test, and last week, I went to pick up my new specs.

I really like the way they look. But there's a problem: the prescription for the right lens is too weak.

Double trouble: I had the glasses made up by a different optician from the one who did the eye test.

My problem now is that I've been sold something which doesn't meet my needs, but I don't really know what rights or liabilities I have in this situation. The fault clearly lies with the lady who did the original eye test - but I only paid £5 to her for that privilege. Meanwhile, I spent a lot more money than that on having spectacles made up from her prescription, but the opticians who made up those spectacles did so on perfectly good faith, and I have no reason to believe they did anything wrong. Getting the £5 back for the original eye test won't really do much to off-set the cost of having the incorrect lens replaced, but it seems to me that I can hardly expect the second optician to absorb the cost of replacing it either.

Did I accept liability for this situation when I voluntarily chose to go to another optician to have the glasses made up? Or do I have any kind of protection against the consequences of having paid rather a lot of money for an incorrect lens? I surely can't be the only person who's ended up in this situation, but I haven't been able to bring up any very suitable advice by Googling. The best I could find was this, which is someone reporting the same situation - but I'm not at all clear that any of the people advising him(?) have any real expertise in the matter, or indeed whether their comments would apply in the UK.

( Further stuff about it, no longer really relevant to the issue of whose fault this was )

Anyway, if anyone has been in this situation, or knows anything about what my rights are, please do comment. And if not, I guess I've just got more inconvenience and another hefty bill coming up...

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