I'll second that, 'theanalyst'!
Just to update you, I contacted Plumbfreeze again two or three weeks ago, asking them to sell me another unit, with a money-back guarantee that it will work as specified, as my original one did have a serious fault.
However, the MD said that he had stopped taking orders now because a considerable backlog of orders had built up, which took priority. (I thought that I might take priority as he had supplied me with a faulty unit, but he would not agree to this! He said he'd let me know.)
He then told me that in any case he might well not be selling any more units directly from Plumbfreeze, because an important sole agency contract was being negotiated with a large plumbing supplier, and they might object to his company selling units. He did not want to risk compromising this contract.
So that's the position at the moment -- I'll contact him again fairly soon (if he doesn't contact me first) and keep you all posted.
(I'm finding it very hard work trying to spend my money!)