Always handy to find an unexpected heat source when you're scratching low down.
Fortunately we don't have too many of these on the North Downs....!
Villarria volcano Chile.
Edit- additional info from Eric....
"However we do have something similar in Sittingbourne, albeit on a (much) smaller scale.
I used to fly in the evening on the grassed-over household tip at Borden that had a couple of 20 feet high chimneys used to burn off the methane generated by the decomposing rubbish. They used to produce a reasonable amount of hot air to supplement the warm air rising from the surrounding trees, however I had to very careful that my venerable Sonata wasn't flown too close to the flame otherwise scorched Fibafilm and doped nylon would be the outcome!!"