I 'm mindful that I need a perfectly circular section at the tips or my joints won't work.
I've bought one of these gizmos so it will cut cleanly:
http://www.screwfix.com/app/sfd/cat/...21946&ts=05537 but am
wondering about where it's been bent and straightened eg to get round
corners.

I think you're worrying too much (!) but...

I've worked with 10mm microbore and you're right, it does have a
tendency to go oval when you bend it. I didn't have any awkward
compression joints to make with the stuff as I used 15mm for rad-tails.
Once out of sight I used soldered 15-10mm reducers and then 10mm back
to the manifold. I am sure compression joints would be less forgiving
than soldered joints if the pipe is deformed. I had no problems at all
with my soldered joints though.

So my advice would be, try to avoid making complicated snaking runs
involving a lot of bending and restraightening as you worm your way
round the corners. You can always make up a series of smaller bent
sections and then solder them together. Soldering 10mm is dead easy
anyway, especially with Yorkshire fittings!

Luke