Sorry for the delayed response - car has been in & out of the workshop over the past week or so.
Fuel trims look OK - additive 1.2%, multiplicative 0.8% (was reset not to long ago).

The EML now stays off for around 2-3 days at a time but is triggered as soon as the owner goes over approx 70mph. She has since switched to V-Power fuel as well as running some injector cleaner through, which although improving general performance has not cured the faults (her husbands idea).
However, the fault codes have now changed - i just get a "knock sensor 1 - signal too low" fault code, & very occassionally its accompanied by a "Random Cylinder Misfire Detected & Misfire Cylinder 1 & Cylinder 2".

I scoped the ingition timing whilst it was idling & although generally stable (say around 10 BTDC), occassionally the timing 'drops' & becomes erratic for around 5 seconds (say to around 2 BTDC) & then returns to the previous stable value. This happens randomly & quite frequently although it does not trigger the EML and the idle speed/stability is unaffected (probably the ICV compensating?).

So, next step is to replace the knock sensor and see if that cures it (my money is still on an injector fault but the owner wont cough up to replace it yet).