Okay then, I've looked it up (and it's a good job I did, because i would have got one of the distances wrong from just my memory

)
The legislation you need is Road Vehicle Lighting Regulations, 1989.
Schedule 1 states that you need a Rear Fog Lamp (or 2 if you prefer!) on a car of the age of Elan (that's not good English

) ie first reg after 01/04/80.
Schedule 11, Part 1 tells you how it needs to be fitted.
I'm not going to re-write it in full, because you can view the original by
clicking here, but the basics are that:-
It needs to be red!
It has to face the rear (between a given set of angles)
It needs to be between the centre line of the car and the right hand side.
It should be fitted between 250mm and 1000mm from the ground.
It has to be at least 100mm from the brake lights.
There has to be a 'tell-tale' warning light for the driver.
The important ones from a legal and aestetic point of view is the height above the ground, and distance from the brake lamps - there must be a variety of ways to do it, but I'll leave it to you to decide how you want your own car to look!