Well, long story short, I replaced the DV, front PCV, MAF sensor and air filter (which appeared to have never been changed). When the car was behaving, it shot off down the road like a scolded cat much quicker than ever before, but would soon revert back to feeling flat.
On holiday recently I had to fill up with BP 97 ron as there was no Shell garage nearby for VPower and the car definately immediately improved. Having stuck with BP for a few tanks, it's now consistently much more drivable and responsive, although there are still times when it really pulls like a train, which highlights the fact that it's not doing so all the time.
Is there any logic to this development, does it point to a particular component fault? Alternatively, does it cast doubts on the software that the car has onboard, should I be investigating what map it's running?