Ok, this is starting to annoy and Google has yet to provide any clear answers.
I'm dynamically creating CSS based on users choice of theme colours. Easy enough, done with PHP but the code is irrelevant here, the problem being that IE is caching the old CSS and upon loading a theme it's not producing the new CSS without an F5 refresh. Firefox, Chrome and Safari all produce the desired effect.
I'm sure it's something simple I'm overlooking but for the sake of my sanity please clue me in.