[pirates]

I have heard even Manchester sometimes spoken that way as well, depending on who you’re talking to.

[NeoTar]

Godmanchester in Cambridgeshire, although now pronunced by most people as 'God-Man-Chest-Er', used to be pronounced (and may still be by some locals) as Gumster.

[JdeBP]

It's not even unreasonable. -caster, -chester, and -cester are related suffixes.

[arrowsmith]

All descending from the Latin "castrum", which means "fort". So if a placename has one of those suffixes, chances are high that it was a Roman settlement.