I bought the one from Lloyds Pharmacy for £9.99 and I've found it is fairly accurate if you use it as directed. I always measure my BP at the same time of day after sitting down and resting for about 10 minutes and it's usually not far off the docs reading. The doc found my own readings very useful as it showed a trend when I first started on BP medication
. Taking your own readings is no substitute for getting it checked by the doc, so if you are worried about your BP, it's definitely worth getting it checked out by the doc.