Thanks Neil, I never thought I'd say it but compared to what I thought I was going to have to pay at Metals4U VAT doesn't seem like such a hardship!

As an aside, I was talking to the wife about the build over the weekend and it occurred to me that box section steel is actually about the same price as wood. 7.6m of PAR pine would set you back about £19 if bought in 2.1m lengths and considerably more if you could source it as a single length.