I have a Discovery 1 with standard cloth seats. The drivers seat has collapsed.

I have an offer of some leather electric seats from a Discovery 1 but are these interchangeable?

I'm not bothered about the electric bits, just want the seats.

Or is there a fix for a collapsed seat? Currently using a cushion to fit in the hole.

As long the are from the same series (200-300) yes. Without the electrics u won't be able to adjust unless u adapt the leather seats to suit the manual function that u have a already.
There'r companies that will refurbish seats or supply the kit to DIY.