Does anyone have a link to the alleged Boots offer at £400? I ordered an Air Wrap last night for my wife, direct from Dyson, for £480. Google couldn't find it for less anywhere.
OP: My thoughts are that you should now leave it.
I’m not a lawyer, but I believe the 'contract' will be with the person you lent it to. Chasing the sister would only complicate things, and possibly indicate that you no longer require recompense from the original lendee and have accepted transfer of the contractual obligation to a third party. Very dangerous.
And under no circumstances meet anyone, or go to anyone's house. You may be accused of intimidation.
It's very simple: Person A (you) lent an item to Person B (the alleged CF), and you can prove you did so. It was clear that you expected it back; it was not a gift, wedding or otherwise.
Person B, despite multiple reasonable requests, has not returned the item.
If it were me, I would buy myself a new one now, then commence a small claim for the financial loss.
Waiting for a small claims order for her to go out and buy you one would be absurd. It will take months, and then you have to chase for her to actually buy the right thing.
Keep it simple, and remove any possibility of your receiving second hand junk. A straight financial claim, evidenced by a dated receipt, plus costs. It’s really easy. If she defaults on a financial award, send in bailiffs.
Good luck!
Mark