Can i ride your bike?

Because i'm 14.5 stone.

I'm worried i'll snap the seatpost if it comes out any further.

I bounce when i cycle, like an internal shock mount system.

But yeah, i'll do some fiddling. :)

You bounce when you cycle? You mean rock your hips side to side? Sounds like your seat is up to high and your actually over extending your legs. If your having back pain from having your torso leant over to far forward obvious solution is to raise your stem a bit, not lower it by getting a pursuit frame which will do the opposite of what you want. Bikes set up for long distance comfortable riding tend to have longer head tubes, and shorter top tubes to bring the bars closer and/or higher and have the rider in a less aerotuck position. It looks like you have plenty of scope for fiddling with your current setup without needing to change frames (except for dropping that stupid basket). For the record I'm over 18stone and have far more seat post showing in my MTB.