Does anyone have a photo of the advertisement that was once (maybe 60 years ago) very common in fields along railway track. Two men in white overalls with a board advertising Halls Distemper. It should be easy to replicate, but were thet carrying anything?

Welcome aboard.
Is this what you are looking for?
https://images.app.goo.gl/RSPQo1fGdnqZm5Mp6

Google hall's distemper boards. Other posters available

@K Reynard
Welcome to the forum
Looks like Port Perran has hit the nail on the head.

There was a similar one near Cudworth, Barnsley for Johnsons Paints.  Sorry can´t find an image of it though.