Swapped original MAF with good replacement - problem still exists.
Swapped another front A/F Lambda - problem still exists.
About to swap a spare throttle body/IACV - will see what happens...

As a side, I've noticed that my front subframe crossmember is pretty seriously corroded which will mean that will need swapping out before the next MOT.

Awesome.

I'm getting seriously sick of this car at the moment...

 

P0171 can be caused by a air leak between the MAF and the throttle body, check the plastic inlet pipe for any splits where it joins the turbo, this includes ALL clips, check the intercooler and pipework for leaks ie stone damage, also check the dump valve and pipes to/from it for air leaks. Anything that leans the measured air from the MAF to the throttle body can cause this, the Lambda sensor by the turbo can also cause it.