Just pasted from UK website:
In order to minimise inconvenience to genuine travellers a grace period will apply during which travellers may benefit from the following concessions:
Nationals of the six countries listed who bought their tickets on or before 15 October 2003 may transit without a visa provided that:
They arrive in the UK before 23:59 hours BST on 20 October 2003; and,
They arrive in the UK by air with confirmed onward bookings by air within 24 hours of their scheduled arrival; and,
They have necessary documentation, such as a genuine valid visa for their destination.
Until 23:59 hours BST on 12 November 2003, nationals of these countries on the return leg of their journey may transit the UK without a visa provided that;
They originally passed through the UK in transit (with or without a visa) on the outward leg of their journey; and,
They started their outward journey from their point of origin on or before 15 October 2003 and,
They arrive in the UK by air with confirmed onward booking by air within 24 hours of their scheduled arrival; and,
They have the necessary documentation, such as a genuine valid visa for their destination.