Not getting anywhere fast I decided to buy some. Specifically some PLA filament from Fabadashary.co.uk (no affiliation).
Once before I had dealings with this company, for their 1.75mm rainbow filament kit. I was happy with the site and the speed of delivery (UK based, so not really surprising). The real plus and my reason for going back for this application is they offer short lengths of filament (minimum 5m). Utterly perfect for my needs. For anyone that doesn't know they hold the widest range of coloured filament I have known a comapny to stock. They were nice enough to include some sample filament along with my original order for some 'mellow yellow'.
Measuring the filament in several places along its length to check consistency,I can happily report a diameter of 2.8mm, with a variation of no more than a 0.04mm.
Now, I am making the massive assumption that 2.80mm is the actual size of nominal 3mm filament. If I am mistaken and anyone knows any better please, please, please comment below, before I waste money on the incorrect inside diameter tubing.
Now I know the filament diameter, I will source some appropriate PTFE tubing. When I get a working setup I'll post my findings. In the meanwhile, if as I suspect others are looking at using 3mm (2.80mm?) filament with their SumPod, due to the lower cost, I am happy to supply lengths of tubing.
SumPod Guinea Pig signing off