Rodge - I would ask for advice at your doctor's surgery. They vary so much. I bought one and had it tested at my surgery - it was way out. I did get a full refund for it and have not bothered to replace it.
I have just been to see my doctor today, and they can lend you theirs for use at home, but it was on loan. I would ask at your local G.P.s, as theirs would be better than yours, and would not cost anything.
I have heard very bad reports of cheap home ones, so I would do this.
For anyone who is interested in the future I have found this one by A & D Instruments Ltd. Their machines are all clinically verified and this one talks. It is model UA-767V available on Amazon. Bit expensive though, but better than a cheap one that doesn't work I reckon.
I'm sure you're right you have to be careful em. But it isn't always convenient to go to the docs and I think these are good, in that they have used them and then tested the results against a manual pressure reading in a large number of subjects, and found them to be consistent.