197 has crank but no start

Hi all, I've been a member for a few weeks so not yet intoduced myself. I'm James and I have a 58 plate 197 that I'm turning into a Targa rally car.
The car has recently developed a non starting fault.
It's a keyless car.
The car will crank every time but it won't start.
I've changed the cam and crank sensors.
I have no spark at all.
I've stripped down the main fuse box (behind the left headlight) I found some corrosion which I removed. The affected area was around the coil supply fuse.
I have fuel at the rail and the fuel pump works (initially)
I have probed the fuse box to discover that the power supply to the fuel pump and the coils initially has 12v then after 3 seconds it drops out. I have tested this at the fusebox, the coils and and the fuel pump itself. To me, this proves the fuel and coil supply systems are shared on a common circuit.
I've checked the engine earth's. All seem OK.
My launch diagnostic doesn't register an engine RPM while cranking but I am not sure this is right or wrong. (Before a try another crank sensor).
I've checked the injector/coil harness and it's all OK.
The spark plugs aren't the best but it has been running great until now.
The "steering not locked" message has appeared on the dash. But I can get it to go.
The car has previously started of a bump start (which in sure it would do again)
This fault first started about 2 weeks ago. Then it miraculously fixed itself (out of the blue) but yesterday it reappeared.
I am scratching my head as to what this issue might be.
Could it be an immobiliser fault?
Could it be a brake switch fault?
Could it be a key card switch fault?
Thanks for patience in reading this long post
Has anyone got any suggestions as to what it might be?
All suggestions gratefully received!!!
Thanks