In reply to Colin Knowles:
I'm the one who suggested to Colin that he try asking here, and having discussed it with him I know what is required. I'll just comment that, as far as I understand it, any climber with reasonable outdoor experience would be able to do what is needed.
I'd have done it myself but it's not terribly local and I am flat out with work!
The suggestion of arborists is also a good one - although this doesn't involve any trees, they would also have the skills required, so Colin, you could try http://arbtalk.co.uk