Prestons of Blackpool were suppliers of mail-order watches for science & industry. They seem to have specialised in Omega and stop-watches. Chronosport in Bath were the heavyweights for Heuer & Breitling watches and sporting applications. The back pages of MotorSport magazine in the 1960s and '70s always had an advert for Prestons, and for a subscription the whole run of MotorSport since the 1930s has been scanned. I did grab copies of some of Prestons' ads a while ago but I think they've been consigned to a back-up memory stick.