Jammy,
i'm in the army and work on the aviation side of things
one thing we use for aircraft tank sealant is something called PRC
try this link:
http://www.silmid.com/prcdesoto/integralsealants.htm
and have a look at it, might help with prices and things
if you do go the sealant route remember its just a temp repair, and you should get the tank changed soon as you can
Also, whe nyou fill it, dont fill it over half way, maybe 3/4s, you'll still get the vapour smell, but at least you wnt get fuel spilling on parts that get hot (rear brakes)
if you have ramps, and axle stands, you could try going for a combination of onto ramps, bricks under jack, jack it up higher, then support with axle stands, bit heath robinson, but if it works it works
You'll also ahve to either drop and swing out, or completely remove the near side lower wishbone too, dont forget to mark the eccentic bolts whe nyou do that so you get the rear wheel alignment right when you put it back together
Mick