Runway Heading
This due to the decrease in magnetic variation as the magnetic north pole drifts with time. It's something to do with plate techtonics.
The scruffy old out of date chart on my office wall says that the average mag variation in the UK in 2003 was 3.5 degrees W and decreasing by 7 minutes per year. I used to use 4 degrees when doing my PPL nav.
But I'm old enough to remember Heathrow having 28 /10 and Gatwick being 27 / 09.
It happens to everybody eventually.