I've had slightly larger dia stuff from Lloyd & Jones Engineers in Liverpool a couple of times. No idea of price I'm afraid because it was for the ship.
They're liable to want to sell you 3m of it though!
I've used a local Metal Supermarket for a couple of metres of 4mm SS rod, never compared prices etc because I don't use enough to warrant it. Not sure whether you'd have a local branch or not, but may be worth checking.
Many thanks for all the replies - there are some very interesting links there.
Strangely enough, I was speaking to a business unit neighbour at lunch time today - he's a metal basher and was asking him if he could recommend anywhere. I'm now the proud owner of 54 metres 6mm rod and just £15 poorer!