The masonry around the flue looks pretty sound, but maybe rain blowing into the flue is a possibility. A leak on the mains side seems unlikely because it's so intermittent.
I think there's some kind of flexible hose above the drip but I've no idea what it does or why it would leak now and then.
Hope someone can throw light on a problem I have with this boiler. Now and then it drips under the main compression joint on the far right and I think the water is coming from somewhere a foot or so above this point.
Had a CORGI round and he could see no tell tale signs of where it might be...