Why are they cheap? Do they use older vans for the conversion? Cheap can mean corners cut, but I have no knowledge of this company, they may be brilliant. Good materials cost money. People who work hard deserve to get paid for a good job done, not everyone is out there to rip people off. I've been looking too and get lots of good feed back about these people http://www.kphorseboxes.co.uk/ Their prices are reasonable for the job, but not cheap.
It may be a complete co-incidence and they may have nothing to do with that firm at all, but personally I would be checking. There were a few more extremely negative posts on that thread, but they have been deleted, probably by the admins as they breached H&H forum rules. I had a box built by that firm and had many issues, all resolved now but I had to deal with most of it myself at extra cost.
Please join the Facebook group Horsewise Yorkshire to share info on this subject , this company has caused allot of problems for many of their customers