Fraser World (
www.fraserworld.com) is running a member-only 3-for-2 promotion at the moment. They have properties in London, Glasgow, Edinburgh and Paris etc.
After signing in, search with promo code MASTER for the special rate. Some good deals here, e.g. £86 for a studio in central London and £67 for a one-bedroom apartment in Glasgow.
Mastercard World cardholders get upgraded to Diamond after one stay, that comes with room upgrade, early check-in and late check-out etc.
More on my blog post here 