I'd expect from £100 to pay someone to do that, shouldn't be more than £500. Depends on the trimmer, the time scale, and the final finish you're after. If the foam underneath is good and it's just the outer fabric (I assume leather, it being black should help keep cost down since it's generally easier to match, if it's vinyl it'll be a bit cheaper still) that needs replacing it should be to the cheaper end, if the foam needs rebuilding it's going to be more. Best bet is to contact some trimmers and see who gets back to you, most of the cost is in the labour rather than the materials on a job like that.
You could DIY it but a professional trimmer is likely to have access to the sort of machine and threads to make a really nice job of that contrast stitching which you generally can't do on a home machine.
Perfect reply, thank you very much. I have bought them now, but not quite ready to use them so they will probably be in storage for a bit. I will endeavour to remember to update this thread as and when I get it done.
Think I paid about £100 for a bolster repair about 5 years ago. Mine was slightly complicated as we couldn't find an exact fabric match, so trimmer had to take the panel from the front part (facing the footwell) and use that piece, and put a similar fabric on the front where it wouldn't be seen. I supplied the seat out of the car to him.
Yours looks easier to match than mine was, but on the flip side it looks like it needs some foam, plus inflation has happened. SO I'd guess you'd be looking at a similar price.