Richard, BWM did try plastic fuel tanks in 96/97. They had a quality problem and the tanks melted. Result cut console out and replace with stainless steel at the manufacturers expense and down time to the owner.
Agreed S/S tanks do have condensation problems, however that aside properly made they will last for years. There are other materials as pointed out in a series of posts last winter however cost and availability is probably an issue.
To solve the condensation problem, keep the tanks full and fit a good water separator. Oh and just use it regularly
Vetus make a tank hatch which has filler, breather and fuel lines etc all in one 6" plate which bolts into the tank and has proved very effective. It means you can then take it out and inspect the inside of the tank.