In reply to featuresforfeet:
Is it the outer or groundsheet that's gone? if it's the groundsheet then I've found treatments not to be very effective but you could just buy a cheap tarpaulin or find a bit of tyvek to use as a footprint. If it's the outer then you can get good results with things like fabsil as it doesn't really need to be totally waterproof to keep the majority of the weather out.
Taping the seams can help things too and it also helps strengthen the seam in cases where the stitching is pullijg or has become abraded. It's worth a go to spend £15 before you spend £2-300 on a new tent.