what tubing should I use any regulations 4 mot I don't have one fitted at moment thanks

Not needed for MOT seatbelt mounts are the area you need to consider. Don't know the wording but they obviously need to be in the right places for safety and strong enough. The body applies a massive tonnage to the mounts in a crash

Correct stuff is cold drawn seamless tubing 50mm diameter have a look at motor sport part suppliers, but anything can be used in reality depends if you are going fully functional safety or a show cage.

Chromoly is the dogs (you know what), CDS is used on most race cars, and ERW is a cheaper alternative.

Common diameters to use are 1 1/2" and 1 3/4".

Wall thickness depends on what you plan to use it for, but safety wise I wouldn't go below 2mm.

Design is a huge part of the strength, because a single roll hoop with no triangulation will fail, no matter how strong the tube is.

Not needed for MOT seatbelt mounts are the area you need to consider. Don't know the wording but they obviously need to be in the right places for safety and strong enough. The body applies a massive tonnage to the mounts in a crash

MOT.... only ever been aware of a tester taking a look at the mounting points to see there is no terminal corrosion and the belt material itself for tears , cuts or scuffing. Have had a belt fail an mot for wear to the material. This was severe and quiet obvious when it was pointed out. Now, I notice, among the multitude of stuff you get saying a bit of corrosion but not significant and so on the is a "get-out" statement saying, if a child seat is fitted in place at the time of MOT then the seat belt\s involved have not been checked. I asked why the first time this occurred... Response.. we are no longer allowed to remove a child seat to look at the belt and then refit the seat in case we do not fit it back correctly and this subsequently results in a child being injured during a collision.   

I remember being told a long time ago by a tester that a MOT is only an indication that a vehicle is roadworthy at the point in time the MOT is carried out and there is no "come-back" from the moment you drive away home.  World`s full of ++++ 

Won`t bore you with actual positions , mountings, fixings and so on in terms of meeting reg`s but if you have any query on this just ask. Roll bar... triangulation... absolute must in something like a buggy.

Just make sure seat belt mounts are good and if you are using full harness they should not be below shoulder height at the back, bolt to the floor and say goodbye to your collarbone in a crash.