So just to bring this thread to a close, I picked the car up yesterday, it absolutely flies now, I must not have noticed it gradually losing power.
Started it this morning from stone cold and no knocking noise, so it looks like the problem was the exhaust. Will update again if the problem comes back.
For info for anyone else who might experience similar, the detailed job sheet from the dealer states that the passenger side (drivers side in US) downpipe gasket had failed , and that he baffles in the back box had collapsed.
It really did sound like it was something terminal in the engine ( as you can tell from the you tube clips)