Hmmm, since my last post I have discovered radarvirtuel.com. From this I managed to find the flight parameters of an Atlantic Airlines B733 (?) flying on an easterly heading over Gransden glider site at between 2,500 and 3,000ft at around 1632Z today.
If the comment above is accurate then the airliner pilots involved are very brave. Gransden Lodge is a notified cable launch site to 3,000' above surface (250').