1-3/8 diameter, 7/16 bore. With ferrules and caps. Will fit all makes.
Can I just say... Make sure you have a shedload (do you see what I did there?) of these to hand http://www.screwfix.com/prods/76030/Hand-Tools/Vices-Clamps/Clamps/Quick-Clamps/Wolfcraft-One-Handed-Pro-Clamp-450mm. (well, a bit smaller and cheaper, but that kind of thing)
@Andy: Righto, thanks for that; I have my Dad coming to assist (read: project manage!) so I imagine that's on the list of must-haves :)