I use Austin Garages too, even though I live in Holland, at least for anything major or needing experience with the 260s (actually near Burton-on-Trent in Staffordshire, not Shropshire). Excellent service and price, in my opinion, but often difficult to get an appointment.
Regular small service (oil, filter, brake pads etc) should be no problem for a good non-260-specialist garage though. Emphasis on 'good', as always! If you can, and I have managed to, try and get a garage to accept that you always provide the parts and materials. They may baulk at that, since most make a lot of dosh on supplying inferior Chinese stuff at dealer prices! Makes sure I get top quality 5w20 oil, K&N filters and the Pagid pads I prefer.
Was there any coolant under the COP in the spark plug tube, incidentally?
David