I do all my own services and it used to be that the big one was 18k but they might have put it up to 24k. Plugs are changed which requires the radiator having to come out and valve clearances are checked and adjusted if necessary. Gets more expensive if the valve clearances have to be adjusted. Cams out and all that. I understand that it is rare for the clearances to require adjustment. Draining the coolant is a pain because it has to be vacuum refilled. Plugs are iridium.
If you are looking to buy one there are a few things to be aware of and they can be found here
http://www.eurokclub.bike/