Result!
Have been trying to get through to the private practice who quoted me £85 just for a scale and polish, but it was engaged all morning, so googled another private dentist who offered den plan, phoned them and they said they were told yesterday they had to take on some more NHS patients (the next nearest NHS dentist is a ferry journey away), so I got myself and the 2 DC registered and now only have to pay £16.50 for a polish! Woo hoo. Lucky DH gets all of his free with the forces.