Ello Govna
It seems strange to be able to give you advice from across the pond on where to get service for youre TDI, but my girlfriend is from Hertfordshire and her dad ran a car dealer (peaugeut) for 30 years before retiring a few years ago. He still knows who is who of service shops so I will make sure to ask him.
Good luck, I should be able to get back to you in a few days.
Cheers
Erik