OK, I've built my J-pole and it's looking quite good. In the end, I
went for 22mm copper tubing as I had some spare. I've still not
decided on the coax though. I've measured the run, and I'll be needing
12 meters (39 feet). In addition to the RG213, I've managed to find a
source of 'low-loss mini 8' (RG-8X). The RG213 is £1.40 per meter, and
the RG-8x is £0.65
per meter. Is the RG213 worth the difference on VHF?

Thanks for all your advice and help.