Learned a few important things:
1.The same fuse works the rear lights and the dash lights. Makes sense really and if I had half a brain I'd have assumed this and realised the rear lights weren't working.
2. Always good to be otherwise driving safely if the cops happen to see something wrong with your car.
3. A bit of humility goes a long way.
4. Don't trust dealer to sell you the right spare fuse kit - check them yourself.
5. Apparently the rear reflectors almost look like the rear lights are on.
If I'd had the right fuse with me and solved the fault on the spot they'd have let me off entirely. They saw me try to fit the wrong type of fuse and only gave me a defect notice and for only the lights. No fine. No points. No examination of the rest of the car. They even let me drive home as long as I rode the brakes from time to time if there was traffic behind me.
6. Hindsight is a wonderful thing, and would have steered me, if I'd been less stressed, to swap a less important fuse into the rear light holder.

7. Perhaps while the coppers were being so lenient, and since, unlike me, they're under no stress when booking a motorist, one of them could have suggested 6?