The fusebox is spotless. Looks like new. I unplugged everything.

The fact that this is intermittent makes it hard to confirm anything. The car is in very good cond for it's age. P0106 (MAP) is the only fault on the ECU. All air pipes, etc. look good.

The only reliable thing I have to go by is when it refuses to start, it will consistently fail, until I press the acceleration pedal while turning the key. Then it's good for the next 20+ starts.

Might be completely chance, but this issue has only every happened on a warm engine. Never happened from a cold start in the morning!

I took a bunch of photos but the 'attach photo' link is disabled :/

Are you sure the engine can draw air? The fault code P0106 relates to MAP Sensor Range/Performance. With a closed throttle the manifold vacuum will be high. Check to make sure that the throttle body is clean and moves freely, and then re-calibrate it but you may well need the assistance of someone with Peugeot diagnostic equipment to help you with that.

Get a Planet diagnostic as that canread live data, not just fault codes.

Google "Peugeot Planet Map" for a map of owners around the UK

Defo the above. With the engine not running the inlet manifold will be at atmospheric pressure which is around 100KPa (1 bar). When it's idling the pressure should be around 40KPa. If when you crank the engine the MAP sensor shows a lower reading than that then the throttle butterfly valve will be closed too much, and consequently not allowing enough or if any air to pass thus preventing the engine from starting. Use your scantool to do live data checks.