jo90..........sorry I haven't able to reply to you sooner.
It certainly isn't an advanced plumbing job to replace the shower valve, but, it does need some experience of these things. Without being able to see it, or note the make, I can't help. I wish I could.
Ask around for a recommendation for a decent plumber. Try your local builder's merchants, or the trade counter at a plumber's merchants. There are plenty of good tradesmen about who wouldn't take you for a ride.
Good luck