Thanks for that parijs, but I seem to have solved it but don't know why. Maybe somebody could explain.
If I open the CD drive that appears when I connect the tablet it shows a setup.exe as well as adb.exe. Running the latter had no effect however running setup seems to have run PCTools which sets up to sync contacts etc.
I do not use this ordinarily and assume that is all it is for.
BUT suddenly ADB Interface appeared in the PC device manager.
Switching the device off naturally causes it to disappear, and reconnection brings it back. So I must have uninstalled this synchronization tool assuming I didn't need it and hey presto, no ADB.
I fear I am missing something here!