You could try Stephen Engineering in Ware, Hertfordshire.
http://www.stephensengineering.co.uk
I thought mine was leaking, and these guys were recommended to me by Chester Transmissions.
I spoke to Jason, who is really knowledgeable and very friendly too. I plan on going back when mine needs servicing. They have all the ZF accreditation and are members of FATE, which means you have a nationwide backup network (albeit and admittedly sparse network).