Hi Rebecca,
Well, as your boar has fathered two litters he has proven fertility and is 'experienced'. This is important for the prospective hirers as they want a positive result. You didn't say if the 'girl/s' he going to are gilt/s or sow/s - if they are gilts it's best that they are served by an experienced boar, i.e. no 'fumbling around' getting nowhere

!
From your point of view, distance for delivery and collection is a cost to be considered be it petrol or diesel, neither cheap these days. So, I would think that bearing in mind all the foregoing, a charge of around £40 (dependent on total round trip travelling) would be reasonable.
After all, getting an AI kit and semen from Deer Park Ireland costs £20 per pig, unless you are already an expert at AI (especially in gilts) it's perhaps not a good idea.
So don't overcharge but don't undervalue either, see how the hirers think about it, the money always comes in handy for routine veterinary services, feed, etc., etc.,!
If your boar was a pure bred of show champion quality and had won prizes and was going to serve pigs of similar qualities it would be a different ball game, the service charges would increase accordingly.