Thanks everyone for you replies.
I'll say straight away that i had not considered the insurance angle – I'll forget all about refilling myself, so any further discussion of the matter is purely speculative.
Michael Gilligan: Yes, Chorley is certainly too far to go! Thanks for pointing me at Wolsely who actually do 2000 cylinders – next huedle is whether they will take a calor cylinder in exchange!
Grindstone Cowboy: No regulator as such on a 340 – but the on-off tap knob does a very good job of regulating the flame. Thinking about it refilling from a larger cylinder could not be done via a regulator as the pressure drop would probably be great enough that it would only deliver gas.
Not done it yet/John Baron: fill level would be easily ascertained by weighing – '340' is the content weight of a 'full' cylinder in grams.
Nicholas Farr/Mike Hurley: the dangers of doing anything with compressed flammable gas are obvious and require the normal precautions – however, in principle I don't see that connecting two containers should be any more dangerous than connecting a cylinder to an apparatus designed to release the gas into the atmosphere for combustion (actually less so, perhaps, as in the latter case flame is involved). Both involve the same number and type of connections.
Mick Bailey: More scope for adventure then. LOL
Cylinders being the 'property' of the company is a sore point with me and I regard it as a con – I've never yet managed to get my 'hire deposit' back on any cylinder of any size.