I have just bought a 1985 Landrover90 which has a truck cab. Is the cab (easily) removable to use it as an open top or to fit a full tilt?

Yes, very easily.

I used to have an old 90 hardtop and we had the roof off that a few times. Removing the truck cab should be a bit easier than a full hardtop.
I think there's a sticky rubber sealing strip between the windscreen frame and roof, so it might need bit of an upward shove to part company.

The only problem we had was that we had to rig up some sort of restraint for the seatbelts otherwise they slip off your shoulders as you drive along.