Here's my take on the situation. I am not exactly an expert, so feel free to hop over to the Mechanics Forum and ask about what's wrong with your bike there.
I think you are literally twisting the frame when you pedal. You need a stronger frame would be my guess.
If I am right, you have some choices. You can get a basic mass produced steel frame and put the parts you have on it. The labor to do this costs quite a bit unless the shop you got it from wants to be nice for selling you a bike that was totally wrong for you.
I think Surly is sold under a different name where you are. But the Surly website will give you an idea of what I mean. Their touring frame is called the LHT and would be a good choice.
If you can swing it, call Thorn or Mercian and have them whip up something for you. Mercian has a huge selection of frame models and steels. But their touring model will set you back about 500 pounds after the VAT. But it would be plenty strong.
The thing about touring frames is that they are very strong, will take a large tire, and have a riding position about halfway between bolt upright and what a racer would do.
Just a thought.
If the shop you got it from will take it back, that might be your best choice. You could use the credit towards another bike. Another choice, were that the case, is
a Mtn bike that doesn't have any shocks. But test ride it and make sure there isn't a lot of flex when you hit a big bump.