Many thanks for the responses.
I understand the European slot issues and I also understand the problem of clogging the airport up with queueing departures. But that was why I wondered why this could not be done in, say, 10 minute periods. So 3 or 4 aircraft could be taken out of the queue, then the departures could have 20 minutes or so to play catch up.
Re the work going on on 27R, a couple of smaller arriving aircraft (shuttles) were slotted in so it didn't seem to have affected them.