Trailer light intermittently working

Hi all, I've recently bought an 200TDI Disco and am very happy with it but have a few niggling problems one of which is the trailer light on the instrument panel.

The trailer electrics all work OK but the light is intermittent - without a trailer it will sometimes come on solid and sometimes flash with the indicators. I can't figure out how to fix it

Does anyone have any suggestions, please?

Thanks

Richard