Thanks for the replies guys. I wasn't being critical of the ATC'er in question...the LHR controllers are more than helpful usually. I guess it could have helped if I'd suggested a flight number swop myself, but I got such an emphatic 'no' to the first query that I decided not to be a pain in the erse about it!
Hooligan Bill, I fully agree with your point on being realistic...but I can't count the number of times I've called company on box 2 advising them we haven't a hope and need a new slot, only to be told quite insistently that we should press on with the first one before trying for another. I think some slot desk operators are just way too over optimistic.