The main host computer system went down for a period around midday. Within ten minutes at Heathrow the outbound delays went up to the hour mark, due to restrictions on departures, and inbounds were flowed (six mile spacing I believe).
The recovery was not helped by Flow Control trying to determine our departure order by allocating and continually changing the a/c slot times, which meant that many aircraft who would have gotten airborne on the 'one every seven minutes' Dover deps or 'one every 4 mins' northbound dep restriction, were now given 2-3 hour slot delays even though the restriction was a domestic, and some aircraft obviously were not ready for their slot, and so their place in the departure sequence went vacant!
Gonzo