I made a decision that putting an oil bag would take away from the bike, so a drastic action was taken and I chopped the back off the tank. this allowed me to weld in a partition to keep the fuel up front and the oil in the back.
Ali welding is getting better every time.
once I got the front all welded up I got the back welded on. When cleaning up the welds I consistently got a hairline crack coming through the metal.
Once I got home and my head clear I realised that what had happened is that the sheet metal had "sunk" near the edges, as i was trying to smooth things off I was going too far down with the grinder, and hence leaving it too thin.
My fix will be to lay a load of beads of weld down to provide loads of coverage, and then file back smooth. Its coming on though. once the tank is finished i'll be trying to make a mudguard with my new English wheel.