Thanks for the suggestions.
All the local +/- 100mm aluminium tubing I can find is either 3mm or 6mm wall thickness (with varying inside diameter) - this would be too heavy for my application.
EDIT: I do see Action Aluminium sell a 100 x 2mm wall (ROT1002) - that could work.
The eBay intercooler solution looks interesting......
The alum. tube that I provided the link for is intercooler tube as you know and its dimensions are OD - 100mm, ID - 96.7mm, Wall Thickness - 1.6mm and 1 mt = 1.5 kg, if these figures are of any use to you.
I have a length here waiting for a project so just measured it to confirm it is 100mm, not 4" (101.6) OD.