if its under eth distributor u poke it on 1 side with a drift (or sumthin that wont poke thru easy) and tap it with a hammer, it should then spin round a little so you can extract it (there only press fit) then pull it out, clean teh hole get another plug (halfords and the like dont usually have em but motorfactors and that sort of place should) stick a little sealer round teh edge and with a suitable sized socket and extension knock it inplace nice and square,
eh voila

dont knock it too hard tho u may damage it or lose it