This was posted by ‘The Wrong Trousers’ on a similar thread 01/11/2015;
UK AIP ENR 1.9-4
9 Inbound Flow Management
9.1 In congested terminal areas serving busy airports, holding can often occur at short notice through the need for the tactical integration of arrival traffic flows to achieve optimum airspace and runway utilisation.
9.2 In the United Kingdom, Area Control Centres (ACC) will not issue Expected Approach Times (EAT) to aircraft when the terminal area delay is likely to be less than 20 minutes.
9.3 Where radar sequencing of traffic from the appropriate terminal holding facility is in operation, Approach Control will not normally calculate or issue EATs to aircraft when the delay is expected to be less than 20 minutes.
9.4 If a pilot requests information on the expected delay they will be given a general indication of the delay, eg 'Delay less than 20 minutes', based on the best information available to the controller at that time.
9.5 Additionally, for London Gatwick, London Heathrow and London Stansted arrivals, when London TMA inbound delays are likely to exceed 20 minutes, inbound aircraft will be given a general statement concerning the anticipated delay based on the best information available to the controller at the time. Subsequently, when the aircraft is within 20 minutes of its original ETA for the appropriate terminal holding facility, an EAT will be issued. London Area Control (Swanwick) will endeavour to frequently update this data to ensure that the information provided to pilots is as accurate as possible. These procedures will remain applicable when ground equipment unserviceability or traffic demand requires the use of an alternative to the main holding facility.
9.6 Operators should consider the carriage of an extra fuel allowance when the flight includes operation in a congested traffic area or where ATC delays are likely.