Calibration Check Sources

Is there anywhere in the UK one can obtain low-activity check sources for calibrating DIY geiger counters (and gamma spectrometers) - something like a 0.1uC Cs137 button? Are any of these things exempt or does it require endless form-filling? I do have the usual active sources (radium dials, smoke-detector tabs, and also some chemicals like Thorium Oxide and Uranium acetate) but of course the exact activity of these is unkown so not much use for calibrating any instrumentation. In the USA and Germany you can just buy these things mail-order but that doesn't appear possible here in the UK - or is it?

ChrisH