flouresant orange signs

[dumbass]

during my travels i keep seeing posts with flouresant orange pitched signs on top, they are at the side of the road or fields i saw one with a number on the other day, can anyone tell me what they are for?

[Panic Button]

I have a feeling they are something to do with making routes of gas mains visible from the air.

[ugly_bob]

I'm sure I've seen one coming into the village I live in, where we don't have mains gas. I was under the impression that they were something to do with water access points for the fire brigade or something - I may be wrong though, and I may not have seen that one where I thought I had.

[ROPESITE]

I believe they are Ordinance Survey markers.