Surgical spirit will do the job, but any harp that has a wood comb will stink of surgical spirit for ever more. The merest trace getting on to the wood will be absorbed and the stink will be impossible to remove. I speak from bitter experience. I don't know how easy it is to get isopropyl alcohol in the UK. I have a couple of little plastic bottles of it that came as cassette head-cleaning fluid or vinyl record-cleaning fluid. A 50ml bottle would last the average harmonica player forever.