I was wondering in a normal BES infrastructure, does it matter what mobile provider you use, are you tied into one only? Is it possible to say, get a SIM from a different provider and continue to use the BB as it was oe would it have to reactivate it?

Also if the above is even possible, is this something that could be prevented by BES admin, forcing you to use a specific provider and preventing you from using another SIM?

No, it doesn't matter which provider you use as long as they provide BlackBerry Enterprise provisioning.

The new device would need to reactivated as the BES uses the device's PIN number, carrier is irrelevant.

The BES cannot lock you into a specific carrier.

So in theory, I should be able to get a new blackberry device & sim on the appropriate tarriff, and use the email and activation code of my BES account to activate the new device with no issues and there is no way of locking blackberry users to using SIMS from specific providers?

Yes, you could get a new device and a new properly provisioned SIM and be able to activate the device on your BES.