Hi Gareth,
Assuming that the leather is badly worn and beyond restoration, a good option is looking for a new seat entirely.
If it is just showing signs of wear, it is possible to refurbish the leather yourself using a good cleaner and also a good leather feeder.
I have seen on TV a seat restoration in which the leather has been "repainted" when very worn, and a good job it was. I think you would be able to find this company online and, from memory, the seat colour can be matched by them and the job done probably without removing the seat.
Let us know how you get on.
Regards,
Peter.