We used the Priory stud - Tiptree. Got no complaints, we got 2nd hand 3.5tn which needed a few minor things doing before a pre-sale MOT. In the time in took for them to get it MOT'd they lent us another box (horse had a hospital appointment!) and were really good.
edit: Just to add, they make brand new as well as 2nd hand boxes!
Priory Stud. We bought one from them, brought it home and found that our big mare was just very unhappy travelling herringbone. There was no problem, we took it back and they lent us a really expensive one to try her in which was herringbone as well but more roomy - luckily she didn't like that either. So they made us one from a box and separate chassis they had with room for two huge horses travelling forward facing and with side and rear ramps. It is fab, with air suspension so that you can drop the ramps really low. Needlesstosay, she is travelling really well in it. They are great people to deal with and build to a very high standard. Here is our box:
~Another Priory Stud recommendation. The guy down our yard has got one of their boxes and raves about it. I went with him to the vets last month and it was really comfy inside for me as well as the horses.
I've seen a few of the lorries thes have bult & they have seemed fine. You shouldn't be far away from them, pop along & have a look: http://www.eahorsebox.co.uk/pages/range.html
We have an EastAnglian Horsebox...one of their more basic models....and we have been quite pleased ..the chassis they sourced is really good...our local servicer said it was one of the best they had seen. If you are looking for a new box on a budget with basic living/toilet cubicle that you can add to later you could do a lot worse.
Thank you all. Am going to have a drive down and look at Priory Stud on monday, so if anyone else knows some good horsebox dealers in that Essex area please let me know so I can have a look while I'm there.
Shall have a google for the other recommendations. Cheers