The Trident CAT IIIb limits were a Decision Height of 12 feet Radio with RVRs of 75 metres at touchdown end, 50 metres at midpoint and 50 metres at far end. The visual requirement for me to say "Land" as opposed to "Overshoot" was that I see ONE LIGHT. I did a few for real and of course hundreds in the simulator on bi-annual checks.
Heathrow had a special radar mounted on the tower in those days and they were capable of talking you to your stand...I used it a few times and it was impressive. It easily got you on to the green lights taxiway centre line and then warned of approaching turn off points. I had to use it years previously on the Vanguard when the fog clamped in suddenly. I think that radar has now gone...It was a white dome topped thing on the side of the tower at the top.