Thanks all - very interesting thread.
The error light is going through an intermittent phase at the moment - it had been on for nearly 2 weeks, then when I started the car up yesterday, it was off and has remained off all of today.
I know that it will come back on at some point however.
By the time I'm looking for the new car it will be a 7.5yr old, 105k miles vehicle. WBAC offered £5k, I know they would try and negotiate down, although the vehicle is in very good condition. I'd like to think a main dealer would offer at least the same against a car valued at around £18k.
I think I'll be sensible about it all really. In the past my trade-in vehicles have been given a look around and I've not been asked any questions outside of "what service history is there on it?" and if that is all I'm asked this time, I can confirm it's had every service it has been due and leave it at that.