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Spent a few hours fettling the Falco yesterday, put new plugs in, tidied up the wiring loom in a couple of places where the malfunctioning alarm was removed prior to me owning it and did a few other bits and pieces the length of the bike. It was well past bothered o'clock by the time I finished so left it overnight until I could get out on it today.

Bike started ok but and I let it idle while I put my lid and gloves on. Kicked up sidestand, clunked into first and went to pull away and then it just cut out. "Oops, bit embarrassing" I thought (one of the downsides of loud pipes I guess!). It cranked but wouldn't start so I turned the ignition off and on and tried again and the bike fired, ran for about 2 seconds the abruptly cut out. Tried again and the same thing happened. And again. And again etc.

Back in the garage and the checked all the basics and where I had been working but to no avail and the same thing happens.

Basically :-

Ignition on, fuel pump primes, bike starts and runs for about 2-4 seconds then cuts out due to no pulse to injectors.
Bike will not restart, has spark but no injection pulse.
Cycle ignition off then on again, bike will start again for 2 secs before feed to injectors is lost again.
Occasionally, the bike will run for longer, up to 30 seconds or more but ultimately will cut out before long.

As I'm getting a spark at all times I'm guessing it's not likely to be a crank sensor issue. There are no diagnostic codes displayed if I connect the diag wire. Anyone got any thoughts?

Have you checked the sidestand switch is working correctly?

You've described pretty much verbatim the effects of a faulty sidestand switch.

Injector relay ?
Starter interlock diode ?
Engine shut off relay also called tilt switch?
You can disable both the stand switch and the shut off relay with tape and pliers

Cheers, I'll get back on again soon. Had to take a day out for the Mrs's birthday. Would the sidestand switch only cause the bike to cut if the clutch was pulled and the bike put into gear, not as it does with the bike idling (briefly!) in neutral on a paddock stand?

Haven't got as far as checking to see if the live feed is being lost to the injectors, tested them by disconnecting one of them and plugging a led noid light in. I presume there is a permanent feed while running and the ECU activates them by switching the earth?

Ok fellas, beers are on me but Fatboy gets a pack of pork scratchings too.
Turns out I'd mounted the tilt sensor back on upside down! Only found out where it was after googling it and my tidying up of the loom left it feeling more natural to mount it the wrong way up!

Woohoo !
I love easy fixes like this. Those tilt switches can be a right pain, it works, take the thing off and then it no longer works.....
Mine is disabled by thin bits of tubing stopping the contact from moving.
Can you get me the Gluten Free pork scratchings ?