Whatever about the merits of moving to Belfast, Mannion was right about the value of the slots.

An estimate of the open-market value of Aer Lingus's total of 21 slots at the world's busiest airport is almost 500m.

A slot at Heathrow is a particular arrival and departure time allocated to an airline. One slot could be an arrival time of 7.30am and a departure time of 8.50am, which means that there can be several slots in the one space leased to different airlines over the one day.

Heathrow does not charge for these slots and the airlines don't own them. However, because they are so scarce and in such high demand, airlines 'sell' them or lease them to other airlines for ever increasing amounts.

A slot's value is critically dependent on the time. A 7.30am arrival time, which allows passengers to get to London in time for work, is far more valuable than a 10.30am slot.

The most recent estimates value each Heathrow slot at stg�20m ( 30m), a sharp increase on the stg�20m Qantas paid for two pairs of slots in 2003. Prime-time slots at Charles De Gaulle airport in Paris are going for up to $50m now.

Heathrow allocates its slots every year to various airlines on what it calls a 'grandfathering' basis, which means that airlines that have held the slots the previous year are automatically entitled to take it the next year, or indeed sell it on to another airline.

Heathrow has a total of 1,200 slots, and British Airways holds 40% of these. British Midland has 12% and Aer Lingus has less than 1%, or 21 slots . . . four from Shannon, five from Cork and 12 from Dublin. Virtually all the slots in Heathrow are in use, which means their value to the airlines that hold them will remain high.

While it is good news for Aer Lingus that it still holds a valuable asset, the possible downside for passengers on this island is that, on a strictly commercial basis, the company can make more money by leasing or selling these slots than by operating them.