I used spg.com to book the Park Lane Marriot and then used the price guarantee as Lastminute.com offered a better rate.

I have just negotiated a 25% reduction with Sheraton Sofia for 30 nights of stays over the coming 6 weeks compared to the best spg.com rate. I asked for the marketing manager and she did me a deal.

I suggest the second tactic and if it doesn't work look for a price match on lastminute.