LHR-BCN-LHR is very similar... My flight was delayed by the better part of an hour, mostly due to lack of slots exiting LHR. Of course, the return flight was also delayed by an hour because the last LHR-BCN flight was delayed on the outbound leg and because of legal reasons, the f/c was 'delayed'.
Oh joy of joys. Thank God I had some slack built into my schedule. Nonetheless the service tends to be reasonably good on that sector.
S.