Thanks guys. To the original poster, my advice is as I posted. Your dealer is liable. The onus is on him/her to prove the mechatronic fault wasn't present at sale, providing you purchased this vehicle within 6 months. You can pursue claims after the 6 month period, but then the burden of proof is on you to prove that the defect was present.
So don't leave it any longer. Follow the Citizens Advice link I posted. Stop using the car (very important), and contact the dealer immediately by recorded delivery letter. Tell him/her that you intend to hold them liable for this fault under the Sale & Supply of Goods Act, and give them 7-14 days to reply. Tell them without a satisfactory reply you will pursue them through the Small Claims Track of the County Court for the total cost of repairs, plus out of pocket expenses.
If VW UK know that this dealer is within the 6 month period they will (rightly) refuse liability of their own.
Don't back down.