Finally there's been a bit more progress on the tank, the bottom and the two front panels are fully welded on, I've checked the welding as best I can at this stage by going into a pitch black room and shining a super bright LED down the welds. I'm still not sure if it will hold petrol but I'll test with soapy water and some air once I've welded the top of the tank on, fuel tap fitting in and tank filler on. I got a lucky break the other day and found the custom fasteners website, they have a weld in bayonet style petrol tank inlet for £4.95 and appropriate tank caps for £9.95, not bad.