Getting one fitted next week, do I need to phone up the insurance company to declare it and will they charge me more? It'll be the only modification the car's got...

Probably not but to find out the definitive answer you'll need to check with your insurance provider to be sure as their conditions all vary. Plus if you start decatting and increasing bore then that's another matter.

Not changing anything, just going stainless. Will phone them up later in the week.