Can't help you regarding Ifor Williams but I took my Bateson Deauville in for service at an independent caravan repairer. The basics are all the same wether it's a caravan or a horse trailer. He did the complete service, brakes, lights, damper, wheel bearings etc & it came to £90.
My 1st year Ifor 505 service was £30 but now bought an Equitrek and the same place stitched me up for £150 - probably as I didnt buy a new Ifor.
Not too impressed.