"Trailer System Service Required" message

[Yorki]

I have a Volvo V50 R design on a 58 plate. About 18 months ago I fitted a Witter detachable towbar (dead easy to do) together with the Volvo trailer module and wiring harness which was activated by my local Volvo dealership.
On recent use of the trailer the error message "Trailer System Service Required" was displayed on the dash information system. I checked the light board and found only the brake lights worked.
The error message will not reset even when the light board is removed and the trailer tell-tale indicator on the dashboard illuminates every time the indicator is used regardless of the light board being connected or not.
I have checked the 12v to the module which is fine. The bulbs and wiring on the light board are fine too. The error message is still displayed even if the module is removed.
I suspect the fault lies within the trailer module or the car network system but is there anyway I can find out other than taking it to the dealer and is there any way of resetting the error message.
Any help would be appreciated.
Thanks.
Yorki

[outnumbered]

looks like dealer to see what codes you have, i have the vct 2000 that can read 2008 models.
mike