Freelander 1 Hi-level brake light.

Can somebody tell me what's going on with this light.
The handbook says 21 watts, the electrical diagrams say 21 watts, so I bought a couple of 21 watt capless wedge bulbs for my neighbour's Freelander 1, and guess what? Yep, they don't fit, they're physically just too big.
I've looked online and found some 16 or 18 watt capless wedge bulbs which are reputed to be the same physical dimensions. Nobody is going to look at the red light in their faces and declare "That's not a proper brake light, it's only 16 watts".
I suppose it all comes down to "Well? It's a Land Rover"

Part number of correct bulb is XZQ100180.

Thanks @wammers

and it has to be 21 watts as per mot and they are a stupid looking bulb

The factory bulb is a capless 16 Watt bulb. I suspect that 16 watts was chosen because it's got a faster response time than a 21 Watt bulb.