Right - update time, cos everyone hates those threads where a problem is highlighted only to never see a resolution.
Was the car scrapped, did it live on the drive for the remainder of earth's days or was it in fact fixed?
After weeks of back and forth with this thing and putting it off because I had use of another car I finally got round to working through possible culprits.
First off was to get that fuel filter replaced - helpfully the fuel filter to main fuel lines under the car snapped upon fitting the new filter but that's fixed and the fuel filter was on. To no avail - still same issue. Relays were checked, replaced anyway with new genuine ones - still to no avail.
I had already done the camshaft sensor so decided to get a new Bosch crankshaft sensor on 'while I'm there' as they seem to be a known weakpoint on these, changed nothing.
Finally got the new fuel pump I had sitting around and decided to crack open the fuel tank and get that in - where I found the culprit, the fuel line from pump to tank cap was original and in a bad way with cracks and gashes all over it. It looks like it was enough to prime and run what was in the line but then couldn't keep supplying fuel at the right rate.
Changed the pump while I was there anyway, changed the line and buttoned it all up.
It's now running *touch wood* nicely and I'll take it for a proper shakedown once I've buttoned up everything under the bonnet and in the boot.