Elvis, I was in this area and I used TR Autos. They're up near the services at Stansted but worth the trip. Good old fashioned Truck MOT place. No facing around. Sensible price and they don't repair as far as I could see so they just want you in and out. Easy to get to but the turn in is small and if you. miss it, its a fair way before you can turn around and come back (learnt the hard way ).
There are other places nearer of course and most these days are incorporated into Truck Dealerships and the waiting list is huge as people book fleets in well in advance. I was staying on Lee Valley Campsite when I went here.
Its a little way away but Oxford City Council Direct services do class 4 for anysize Mo Ho , are good to deal with , and will even sort any probs if you need them , but dont look for work as they are too busy with their own Fleet . They do mine , no complaints about 60 miles
Just a quick update I used Royton at Redbourne, arrived at 3:00 was done by just before 5:00.
Good service commented on the rubbishness of the headlights and helped jury rig a problem with a connector at one of the brake lights (new connector on its way from the US).