It may well be 'splashback' that is responsible but sealants from brickwork are not something I know much about, certainly not enough to make a recommendation. Most of those I've seen strike me as being a bit 'gimmicky'.
Thanks. Jim, great idea. Some clear Silicone and then monitor to see if it is splash back. I hope it is splash back as then at least I can deal with it. Certainly seems like it is.