Is it possible that this sling can 'just come off'?

Only if it was misfitted.

So tell them they are trying tto confuse a simple car owner and no doubt the SMall Claims Court will see it differently..

Write a letter to the garage that screwed it up, giving them, say,14 days, to sort it out or you will issue a small claims court claim-send it registered post, don't bother with the telephone or face to face, get it all in writing.

Remind them that you will claim for the court costs and all reasonable expenses on top as well. If they don't respond or refuse to fix it get another garage to sort it and issue a small claims court claim for the cost to the original garage.

I'd be worried about the money, i doubt a CRV rear bumper is a tuppeny h'apenny item.

You've had good suggestions above how to sort this out, you can't argue with idiots they'll always drag you down to their level, much better to have a judge explain to them the facts of life, they won't be rude to him/her unless they're really stupid..

You don't need back-up info "to give to the court". Just write it all down in plain language, don't try to sound like a lawyer, keep it brief, tell the truth, enclose as much written proof as you have, don't waffle. I would bet that the garage will settle before the court case as soon as they realise that you know what you are doing and are serious about the claim.