Fingers crossed that you've found it, but I have a suspicion that that squirt of contact cleaner won't be the last you'll see of that problem rory.
If there is enough of a fault with the wiring (high resistance) to prevent the electrovalve energising properly the ECU should (usually does) pick up the lack of current and goes into a failsafe mode where it switches both electrovalves into hard mode.
This will cause the rear of the car to also go hard. The fact that the rear didn't go hard and the Lexia didn't report a fault suggests that you might have an intermittent version of the faulty diode issue, as a faulty back EMF diode usually isn't detected by the ECU.
The front electrovalve on my previous Xantia had a dry joint on the diode within the moulded coil packaging that was causing the connection to the diode to go high resistance when the engine bay heated up. This was causing the electrovalve to intermittently fail to operate while driving when the car was hot but it usually passed the bounce test when stationary! It took me a long time to pin this intermittent fault down and prove it conclusively.
If you haven't done it, definitely do the diode mod, if only for insurance and peace of mind...
