David,
I store at Watford Gap near Daventry and for the last 3 years I have had Solihull Caravans Ltd come out to do my service on site and am well pleased with their thoroughness, punctuality and quality of work to date (got them booked again this year already)
Advantage for me as the owner of a four year old Bailey is that they are Bailly approved and so fulfill the warranty obligations for the continuing damp warranty. Last year they charged me £140 for service and damp check, although they found a number of other items which had failed and so pushed up the bottom line cost accordingly. Took their two guys about 2+ hours and one advantage with sticking around whilst the work is done is the amazing amount of technical advice you can pick up from them as work proceeds. Their phone number is 01676 540056. But if you are interested dont leave it too long as I know that when I booked they had only a few dates left this spring.