The air con compressor has failed on my 1990 2.1 td,does anyone know if I can fit the belt suited to a non air con engine to my engine, which I think will be a shorter belt to take the compressor out of the loop until i source a replacement compressor or is another idler pulley used and retain the original length belt?

Yes, you can do it. I'm not sure about the length, but it's the same as non aircon XMs.