Hey Bitzer, it's been a while. Hope all is good across the big pond.
Your garage roof beams should be fine to support the weight, but, if you are unsure, take some 4x4's (that's what we call them in the States--because they are 4 inches by 4 inches and you can get them 6 feet long, 8 feet long, 10 feet long, or 12 feet long.
What I'm gettin at is simply take some boards and place them vertically up to the roofbeams for extra support. You can get one to go horizonatally up against the existing beams too if you desire. With that kind of support, you will have no problem. Here in the states, all that would cost under $25.00.