Might be useful for others.
This is the tube I used from ebay.
http://www.ebay.co.uk/itm/3212065946... 4.m1423.l2649
I think 3mm wall thickness if fine, from recollection the triangular section further up the column is hollow, smaller diameter and similar wall thickness.
I've only tacked mine up so far, but reading this thread I may put a roll pin in once fully welded but need to find someone with a more powerful MIG than mine to do the full welds.