I just took them back to B&Q who found they no longer stock them, and I was going to contact the manufacture as it stated in the instructions to see if we could send them back for exchange but the B&Q assistant pointed out that as they are impossible to disarm (which is one of the requirements) they could cause mayhem at the Post Office not to mention the effort of packaging them up properly and paying for postage, so I took his offer of a refund instead.
I just searched for replacements online, the manufacturer's site lists stockists, but the plug in smoke alarm is not listed with any of those stockists or on the manufacturer's site so it seems they are discontinued and I wonder why? I thought I had found two in a faulty batch, the principle is wonderful - no batteries required ever and a 10 year guarantee. I thought this would be so safe.
So I would not be in a hurry to buy from anyone but an approved stockist, because that faulty batch may have been much wider than one batch and this is what you could be getting from cut price merchants.