I was at Chivenor when the crew flew the bendy diver under the Tamar bridge; not my job but I did fly the reconstruction for the BBC 999 programme that followed. The diver had surfaced on the North coast of Devon, around Ilfracombe I think and the lowest route was planned which ended in the Tamar valley en route to Fort Bovisand where the cab landed on the chamber roof which was quite tight for a Wessex! The advice at that time was to stay as low as possible and everyone presumed 100 feet was a good height. As we now know that is not exactly required but the important thing is to try and maintain your chosen altitude but anywhere up to 1000' is now considered reasonable.