As Tony makes it look so easy, I thought I'd have a go at hand cutting some Indian Sandstone flags that I'm laying at home.
I found a supplier called Axminster Power Tool Centre http://www.axminster.co.uk/ who supply a set of Mason's chisels - Pitcher, Bolster and Chisel - made in Sheffield, by a firm called Ansell. Very prompt service and you can order over the phone rather than the internet. The tools seem good quality to my inexpert eye and are hefty and sharp.
And to my surprise, it worked - first time! So far I've managed to trim a couple of inches off the edge of an 18inch flag, and shape another to fit in an angle where two walls meet.
I will say though, for anyone trying this for the first time, don't expect the stone to just fall off when it sees the pitcher. I sit behind a desk in normal working life and tapping at a keyboard doesn't exactly give you the muscle tone required for bashing away at a pitcher with a club hammer, but patience, perseverance and a good aim paid off. So - when Tony says 'firm taps' he means it (just be prepared to not be able to grip anything firmly for a couple of days afterwards).
Excellent site Tony - thanks very much for the very clear and concise advice.