Being something of a lion on internet forums and a mouse in face-to-face confrontations, I returned to the shop this morning and pointed out the lock was unsuitable because I needed something from the Sold Secure range. Suddenly the chap was very knowledgeable about the requirements of insurance companies so, after trying to sell me the hugely expensive one again he said "I think I might have something downstairs" and returned with a Kryptonite shackle lock with the necessary silver sold secure rating.
Not quite sure what to make of that tbh
It cost me an extra £5 and is no doubt a lot cheaper online but, purely to support independent retailers, I also paid £40 for a pair of Gore Windstopper gloves.
However, I doubt I'll go back. I've coughed up £80 for goods I could have got for £45 online and I feel I've been misled, messed around and inconvenienced.
Being a terribly nice British chap I didn't complain or make a fuss in the shop of course.