As far as I understand it a Class 4 is a light van and Class 7 is a commercial. I'll try and get in touch with a pal who owns on MOT station for clarification.
Good job you put it in after all then ogri. The 6 months extension wouldn't have been worth sailing through a stonewall with a pallet of bricks on the back for sure. Doesn't bear thinking about. Same reason I put my car in, as there's not many places to go if it all goes wrong on that steep hill at Symonds Yat when I go fishing.