I've just had a similar problem, I opened the relay pack (it levers open with a screwdriver) and found that water getting into the box had rotted one of the legs on one of the relays, this wasn't obvious until I cleaned the board and the leg disappeared.

Soldering in a short wire jumper did the trick and the car starts faultlessly now.

Have you got the 5AS immobilser (with the standard Rover fob) or an aftermarket immobilser, or both?

I had a problem with my 5AS. It was clicking happily under the dash when you pressed the fob buttons but the message clearly wasn't getting through to the ECU as the car would start but only run for 5 seconds then stop again. This would happen repeatedly. This is a what an armed 5AS does apparently.

I fixed it by taking the dash top off and finding the 5AS and wiggling the wires - it then worked fine so a stripped back the connections into the harness and found a dry joint.

My other immobilisor is a Toad - this breaks the starter and fuel pump circuits - if you dissarmed the Toad before the 5AS the fuel pump whirred even if the 5AS was armed.

I've got a 5AS & key fob, but don't think its that.
I think the central locking is triggered by the 5AS & that is still working. Also it starts from a starter button which illuminates red when ready to go. If immobilised, the button doesn't light up or the engine turn over. At the moment its lit up & turns over.
Going to pop over to a Rover breakers tomorrow who have a couple of relay packs. So will try one of those first. If that doesn't work then i'll investigate the immobiliser & ECU further.

Went & got another relay pack this morning. Plugged it in & no difference.
Started checking the wiring further, wiggling wires around the 5AS & the ECU. After wiggling the wires around the relay pack & ECU, it suddenly burst into life. So somewhere in that pile of spagetti, i've got a dodgy connection. Problem was that i couldn't get it to fail again, no matter how much i wiggled them! So i'll have to go through the whole lot.
So ordered some new connecters & heat shrink today. Not my favourite job doing electrics ( give me a big spanner & hammer anyday ) but it needs to be done. I knew this needed sorting but i've just been putting it off too long.



Anyway at least she's running again. Thanks for all your suggestions & advice.

You can sometimes get that with a poor connection, they oxidise, giving no connection but a wiggle (technical term) and they clear the corrosion and are fine until the next build up. Regular use is probably your best bet for it to not fail.

And tidy the wiring up, you'll get wires snapping through work hardening if you let them move around too much.

If you can, check the Earths first, a bad Earth is a very common cause of electrical problems - especially on a none-metal car.

At least you've made progress. A car problem is bad, an electrical car problem is worse & an electrical car problem that you can't find the cause of is a nightmare!


SAS

It turns out I spoke too soon.....

No sooner had I fixed the relay pack it began to refuse to start altogether intermittently, waggling the wires seemed to do the trick so I came to the conclusion I either had a duff connection in the loom or the main plug was at fault. My MEMS unit is one of the old original versions from the early 90's and the plugs and sockets are available from PoleVolt ( http://www.polevolt.co.uk/acatalog/ECU_Connectors.html ) so I bought a set and replaced the loom and ECU connectors.

After doing this the car ran brilliantly for 24 hours and then started refusing to start again, sometimes the fuel pump would run, other times it wouldn't, when it was refusing to start there was no spark. I opened the loom and found the leads that run from the ECU to the relay plugs to supply the fuel pump were sharply kinked in several places, I don't know if this how they were in the production loom or how they have been left after 'modifications' but I had some spare ECU contacts left over and I replaced the entire run of the wires.

Hey presto! The car runs again......... for 24 hours. I closed the garage and walked away for a couple of days.

Having spent some time studying the engine management diagram I started going through the ignition side with a multimeter, I had continuity through the wires, the coil windings were fine, there was a supply to the coil but no dwell when cranking. Testing the crank position sensor showed an open circuit, after removal it showed the expected level of resistance which is odd as I'd always understood these things either worked or didn't. Replacing the sensor has now fixed the starting and running issues.

If I'd noticed the rev counter wasn't moving when the engine was cranking I could have saved myself a lot of time and heartache over the last week or so.

Experience, the arrival AFTER You need it

Yep, though to be honest I think the rest of the stuff would have ended up being done one way or another anyway. The next step will be to replace the engine loom wiring with decent thin wall stuff with new connectors and heatshrink protection.

Had a similar problem on my son's Corsa a couple of years ago that turned out to be the crank position sensor. It always worked fine when cold but sometimes would cut out and then not re-start when hot. When it had cooled back down again (several hours) it would start and run fine again. It seems the sensor undergoes some kind of change as it gets hot - not sure if its a physical change due to expansion or it changes its electrical/magnetic properties but it seemed to be a known issue when I checked out the Corsa forums so I started out by replacing the sensor (only £9). Completely sorted it and worked fine for another year at which point the car was sold on. I guess these types of sensor are similar in their properties for most cars so not unlikely that similar problems would occur on other makes too. Glad you got yours fixed - good detective work!

This one was the other way around, it wouldn't start when cold, if the car had been stood outside through the day and been warm it would often start and run. Of course you tend to only notice these patterns when you look back at it. A new Lucas sensor hit me for £32 from the local motor factors, still it could have been worse, I've seen them advertised online for £75 at a specialist supplier.

All this has made me finally start soldering up the Megasquirt ECU that's been sitting here for ages, I have a feeling I'm opening up a whole new bucket of weird with this one.