They are, in fact, external fuel tanks, Kit.
I'm pretty sure the British still plumb theirs into the fuel system, as they did with the Matilda in WW2. The Soviets/Russians didn't/don't.
The British system is better, it means the tank can run on the external tanks until they're either shot up/off (if they're shot at all, being on the rear of the tank) or empty, after which they can run on the still-full internal tanks.
The ex-Soviet system means you run the tank on the internal fuel tanks until they get low, then you have to stop & hand pump the fuel from the external fuel tanks into the internal tanks, if they haven't been shot up/off (which was more likely as they were, generally, carried on the sides of the engine deck instead of the rear).
In both cases; diesel doesn't ignite very easily but, if it does, the fuel is outside the armoured hull & the burning fuel just drips/runs off onto the ground.