Hi Kevin
Jowett vehicles were made in Bradford and they used that name for their commercials, much like Vauxhall used Bedford as a distinct name for commercials ( I believe they were made near Bedford, the town, as well)
Come and have a look at this site
http://www.oldclassiccar.co.uk/ if you want more info about Jowetts you could always join in the forum where you will find enthusiasts for all sorts including Jowetts, or just email the Admin and he will post any questions you want to ask.
Nick::
Trojan was a separate company based in Croydon, Surrey
http://www.trojanmuseumtrust.org/