Graeme opined the following...
Buy the welder. It might cost more up front, but just think of the sense of
achievement. I take it you checked out that mitring program I mentioned?
I've not got any bikes to make, but I tested it out on a few kitchen roll
tubes and it works pretty well :-)

Yup. I remembered seeing it before but not using it as I was working
with square tube!

WRT welding. Telford college is running an evening course on MIG welding
(And another on TIG which'd be fun, but less useful) starting in
September which I'm hoping to attend.

Jon