Front doors are pretty easy as they have 4 axes of adjustment, rear doors have only 3 and the missing one is that which controls the height of the rear end of the door. To get this right you may have to deploy a large hammer and a lump of timber to the fore and aft fit of the top hinge. At least, that's how I did it! A more technical solution would be to use shims but i've never seen any.
The fit of the rear door to the rear quarter panel is poor at best and tends to get worse the newer the car is. By the time the last few thousand cars were built, the press tools used on the panels were worn out, in fact, several years past their replacement date. But the car was nearly obsolete, SD2 was supposed to replace the Dolomite range in circa 1978, it's cancellation meant the car (and the knackered press tools) had to soldier on another 3 years, nobody was going to invest many thousands in tools to build what, in manufacturing terms, was a handful of panels/cars. Add in the possibility of (used) door replacement and wheel arch repair panels fitted and your chances of getting this gap spot on are negligible.
Even in the early days this shut line was problematic, see the production line pic below for an example of how NOT to do it!
So take heart, bad shut lines was always a Triumph quirk, if the shut lines are perfect, it's 100% guaranteed not original!
And this was my own Vermillion Sprint, as bought in 2012. Rusty but unmolested and still with all 4 original doors.
The gap was so awful I questioned the rigidity of the shell! And got told the same thing, "they're all like that mate!"
Steve