Post crossed with your last, but...
Oakey, if your filter hasn't been changed in a while then start with the plus gas early on the flare nuts. I tried pretty much everything on mine - my flare spanners were too thick to be able to access the union , and then eventually I had to use an induction coil (depressurised rail, and fire extinguisher close at hand) and a dremel down the side of the nut on the filter housing (both sides), before the joint surrendered. Just don't go nuclear too early - once you've stripped the nut then you're toast. And it's an Imperial fit too, btw - or at least that will give you a much better fit. That said, I doubt that would be the issue - perhaps an injector has gone bad? I sent mine off and had them all cleaned for 80 quid IIRC
Re Noid lights, I suspect you can derive just as much information by disconnecting the injectors one by one and detecting a change in engine speed/ tone (or not), although I don't have any so may be talking b***s