OK - strange thing happened to me the other day so I'm wondering if anyone else has ever experienced this.

08MY TDV8 towing large but empty box trailer - drove fine for an hour.
Parked up for 40 minutes or so, got back in and started it up
Instead of idling at around 750rpm, it was idling around 500rpm and pressing the throttle made not a bit of difference.
I could engage drive and reverse and could hear / feel it was doing so but still no effect when pressing the throttle
I stopped it and restarted it a number of times, even got out and locked it / unlocked again with no change
Finally, as none of the warning lights on the dash were lit apart from the green "don't forget you have a trailer attached" one I decided to unplug the trailer electrics.
Bingo - started fine, idled properly and all working again
Plugged the trailer electrics back in and no issues.

Sounds like you've just re-initialised the trailer ECU by unplgging/reconnecting it.

Are the trailer electrics and socket in good condition, corroded sockets can do all sorts of strange things, to the vehicle lights on an old car, so on something computer controlled like the RR i would imagine there is more scope for problems.

Thanks Zirconblue,

So is there a separate trailer ECU?

All the electrics and sockets are good - I only had them installed last October when I bought the car - so that shouldn't be the issue.

There is yes, it detects when a trailer is attached and changes the way the car behaves, locks suspension to the standard ride height, changes the way the DSC works, changes the gear change map, checks trailer lights, inhibits the parking sensors and fog light etc. At least it should have but of course some cheap tow bar fitters just use generic bypass relays.

I agree if the electrics are reasonably new, then it shouldn't be an issue with corrosion, but it's worth having a look. The older 7 pin sockets are terrible for it, and can rot out in the backs of them as well as the front (which you can see). If the wiring on the actual trailer is iffy (ie tail lights full of water), it can have the same effect as a rotten socket when you plug it in.

My mums old Citroen used to have battery problems after being driven in the rain - turned out the trailer socket was filling with water and causing a drain. Took it off and cleaned i all up and all was good.

Very interesting: it should all be kosher genuine bits and to be honest it does feel like it behaves differently when the trailer is plugged in. I'll have a good look through the trailer and the Rangie wiring - one item that could be a weak link I guess is that I fitted the Euro socket to the Rangie and then have an adapter to the 7-pin electrics on the trailer...

Thank-you again for sharing your knowledge