Living in the U.S., I can't make any recommendations, obviously. But, considering the size of your material and your proximity to the Thames, I'd at least suggest phoning a pleasure boat repair and maintenance company. Alternately, I'd call an automobile exhaust pipe company (here in the U.S. one would be Midas)... they have several sizes of tubing rollers for such bending jobs.
I've bent 1'' OD 6061-T3 on a jig I made. Basically, it's a wooden device made of plywood with a slot the same size as the OD of the material and in the shape of the bend. Working with a lever in one end (the tighter fitting the better) of the 5' long piece, it was fairly easy to work it into the slot. The tight fitting slot keeps it from buckling the sidewalls.
Filling with sand helps. but it's difficult to get a good, tight seal on each end. Even then, one would need a jig for the shape of bend. The one's I've bent have been about 5:1 ration.