Sudden non start hdi

[Poodle]

I think by the fact that it runs and starts that I can rule out electrical fault or immobiliser

An electrical fault is if anything more likely to be intermittent than a mechanical fault, it's half the reason they can be so hard to trace.

The lift pumps are generally quite noisy when you pull the seat and sound insulation off the top, doesn't mean it's knackered.

I'd suggest getting peugeot planet on it and see what it says, no point in sitting there guessing if you've got the means to interrogate the ecu. Remember reading the codes is only the most basic function of pp2000 when it comes to diagnostics.