OK, I'm in the trade, and reasonably conversant with vehicle electrics. 
We have a 54 plate Fabia VRS here, it's a car I use for commuting, approx 100 miles a day. Up until about 4 weeks ago faultlessly. 
First indication of issues was a sudden refusal to start in the morning, it would crank but not fire. This happened a couple of times, most of the time it would start fine, it would run fine, but sometimes it would just crank. Then a couple of times it died whilst running, once when actually driving. 
A code read pulled up codes relating to the ECU control relay. I can't remember the actual codes offhand.
We replaced the relay, it was cheap enough from TPS, and a google suggested others had had similar problems. It seemed a good place to start.  
The car was then fine for about three weeks until this week when again it failed to start. A code read wouldn't connect to the ECU, and in just about every other system, there was code for lack of communication with engine ECU. A quick check of the ECU live feed pins, #1 and #2 showed there was no power at the ECU. with the relay out there's power at pin #30, a temporary jump lead between pins #30 and #87 and the car starts. 
A new relay was ordered and I drove it to work today, some 45 miles with a rigged relay bypass. The new relay has just arrived, been fitted and......Nothing. 
So, the relay in question is #6y0906381, marked 469. It's in relay position 15. It has 4x pins, #30, #85, #86 and #87. #30 is obviously live feed in, #87 is out I assume directly to the ECU, #86 is unused. #85 is therefore the switch ground. Only it doesn't. I've tested the back of the wire under cranking and it remains live, ground it out whilst cranking and the car starts. So, the big question is, does anyone know where this wire goes, and what grounds it to allow the car to start. The wire in question is blue and yellow. Thanks in advance for any info you can offer.

ASZ or BLT engine code?

It will go to the engine ECU I'm pretty sure, but engine code will get me to the right diagram quickest.

It's an ASZ, right? Pin 18 of 81-way connector at engine ECU.

Brilliant, that's just the kind of positive factual help I was after.
It is an ASZ, I did wonder if it might be ECU,

Ok then so if that isn't earthing out on cranking/running, can we assume the fault is internal in the ECU?

 

Or the wire is bust somewhere on its length.  Get an extension lead on a multimeter probe and continuity-test it from ECU connector to relay socket.

I will do. Lunch first!

 

Wire breakages are much more common in these cars than electronics failures, so I'd be optimistic of a cheap fix.  Please let us know either way.

That's good to know. I'll be back in a bit with an update.

 

Other possibility is that the ECU thinks it shouldn't be letting the car start, and so isn't pulling that wire down because some other fault exists.  Might not be an ECU fault as such, but some other input that it isn't happy about.

Bingo, problem solved. It was a break in the wire to pin #18, I spliced in at the ECU end, and engine side of the bulkhead, it's all back to normal now. Cheers for your help Sir, very much appreciated.