Afterwards, I asked the controller what was going on. Turns out that although he can clear me to land after an aircraft, he's not allowed to clear me to land after a vehicle. I didn't know that!
That's because, in the case of the aircraft, you're both going in the same direction and the intentions of the aircraft are known (he's going to vacate, and you have good enough two-way comms to ensure he does what you want.).
The vehicle on the other hand is an obstruction, probably without the same level of communications - and the rules state you cannot land on a runway which is obstructed unless you 're-declare' the distances available. The rules have to be written for worst case scenario - you can't have one set for the aircraft that can land in 600m and another for the aircraft that requires 1500m.