Well I've now received another email from the Director of Revenue Strategy for the Marriott properties in Prague:-

Dear GlennTheBaker,

We are sorry to inform you that the Renaissance Prague Hotel will no longer be operated by Marriott International. Therefore, we are not able to accommodate your confirmed stays in this hotel. We have been working on finding alternative accommodations and we are happy to inform you that we have modified both of your reservations and changed to another hotel in the city center of Prague under the same booking conditions. Your reservations is at the Marriott Executive Apartments. (http://www.marriott.com/prger). We are certain that the friendly staff at the Marriott Executive Apartments would do everything possible to make your stay memorable.

So it would appear that the hotel really is leaving Marriott. I've emailed back to say that I'm not happy at having my reservations relocated to a totally unsuitable alternative and that I should be accommodated at the Prague Marriott instead. Any tips on what to do if the answer is 'no'?