Hi Gordon,
If you take the B833 from Garelochhead, which is a public road, you will get good views of the base from across the loch. Above this on the peninsula is an MOD road (on MOD land) which leads to the main gate at Coulport. Public access is permitted on this road which starts at the roundabout at the junction of the A814 and the B872 and this will afford you elevated views of the base. In both cases, a reasonable telephoto lens will produce good results.
The MOD Police are used to occasional ship photographers and they are well aware that as one, you will see less than Google Earth, never mind all the undeclared spyware that is above the base !
In this part of the world your biggest challenge will be THE WEATHER !