just looked up my gran prix winners book and theres no mention of clive horton so i take it the 125 tt wasnt a round of the world championship that particular year, meaning that tommy robb was indeed the last british winner of the 125 british gp. so cox and parish were right! and that spanish gp win for mortimor was 1973, your right tel.
sheene never won the 500 race at silverstone so it looks like tom herron was in the last british rider to win the 500cc british gp(1976 senior tt). im trying to work out who won the first british gp to be held at silverstone, maybe there wasnt one in 1977? if thats the case then it was kenny roberts who won it.