A dearth of responses....
So , I'll carry on in case it helps someone equally stubborn one day.
According to Chris at splatshop the 1409 xl tank is made from the same plastic as the original. The same question went to sherco via my dealer, no answer yet. Chris thinks a new tank will work ok until it too degrades. (My original tank is pooched) I have ordered a 1409 xl tank, ~$250 cdn which isn't too bad as far as tanks go. I'm 99% sure I'll "line" the new tank with caswell tank sealer, it seems to be the only convincing option for plastic tanks. Hopefully I'll have enough sealer to do the original tank too, it needs extra help due to various failed "repairs", hope spings eternal. If all that fails I'll fabricate a tank
I'll update here one day...
J