When I first came to Northamptonshire (usually called Northants), the local radio announcers kept talking about this town called "Toaster". Took me a while and a quick squiz at the map to realise they were talking about Towcester. Which led me to look into the pronounciation of -cester names. Turns out there's a simple rule: If the -cester is preceded by two syllables, the -cester is pronounced as spelled, as in Cirencester - sirensester. But if the -cester is preceded by only one syllable, it becomes "-ster", hence Leicester - lester, Bicester - bister, Gloucester - gloster, yadda, yadda.