What is currently the law on driving robin reliant cars?

I remember hearing once that you only had to have a provisional to drive a 3 wheeled car, does anyone know what the actual law is?

No I believe that you're slightly mis-informed. As a three wheeled car with a weight under 453.6kg a (correctly called) Reliant Robin is classed under catergory B1 on your licence, meaning that if you have a full motorbike licence, you can drive one of these in the bad weather.

If you have a provisional licence you CAN drive one, but you need L plates and a full UK licence holder (of min 3 years and aged over 21) sat next to you :)

as far as i know, unless it has been very recently changed, you can drive one on a provisional license.