Sorted :)

All sorted. Thanks. 

sounds like crank sensor to me? if its runnign the abf coil then that could have packed in, i bin these and convert to use mk1/2 coil.

have the diagnostics been wired in, if so check them

I don't think it will be the immobiliser, like you say it should at least try and start then cut out. Thats what mine was doing when the immobiliser wasn't quite right.

Follow rubjonny's advice

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hmm coil power would be the 1st thign on your list to investigate, this is powered straight from the fusebox which in turn is fed straight from the ignition switch.  check the ignition switch wires, make sure there are no bad splices from alarm/immobiliser installation or whatever. the black wire is the ignition live feed, red is the live feed.

a quick way to bypass a faulty ign switch/wire for testing is run a wire from battery + to the coil +, if car startup fine then you know the problem is with the ignition feed somewhere

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is that a green wire wedged under one of the relays?

there may not be an ecu relay it depends how the loom was made, i cant see a fuel pump relay either…

That is a green wire, I don't think it's wedged though.
It's the first time I've looked at the fuse box since I've had the engine conversion done by a garage last summer. I noticed there wasn't a fuel relay too. Not sure of the implications of this because it ran fine before this hiccup.

I think it's a bit beyond my amateur guesses now so will have to take it in somewhere.

There was me hoping it would be a simple relay.

trace your engine loom see where it goes, maybe they put it all somewhere else? could have added the mk3 fusebox to run the engine, not pretty but it can work

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the tci wont cause the immobiliser fault, but that may also be a separate issue.

if the immobiliser kicks in the car can still start for a few seconds so that isnt what is stopping the spark

What could cause the ecu blocked fault?  Should I look into having it defeated or replaced with a non immobilised one?

presumably the immobiliser box was wired in, if so this could be the problem, you should be able to scan the immob box with vagcom this will tell you what the issue is.

or it may be the ecu immobiliser was defeated, if so then i think it is normal for a fault to be shown even if the car starts ok

so basically get it sparking first then see where that leaves you

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try your old crank sensor maybe, plus check over all the engien wiring carefully. its possible for new crank sensors to be faulty too especially if you buy cheap parts