Thank you everyone. I will have my flight contact Eastern Radar and leave Brookmans Park to complete the air test over Norfolk and the North Sea between Red One to the south and Blue One to the north. Does anyone have any idea of what VHF frequency would have been used in the period 1983 to 1988?
It is a long time since I did any air tests and all these details are now long forgotten! I have done many training flights at Bedford on 707s and 747s so know they can handle a 747, thus an emergency return to Bedford would seem a good option (as corroborated by eckhard and Talkdownman), especially as the captain flying the air test is likely to have been familiar with that airfield.
Danny, that must have been scary but, then I expect you had many far more hairy things to cope with in those days! Boeing said that the 747 would be controllable after the separation of a complete flap section, but they said nothing about the controllability if the hydraulics were compromised.
Chevron, I agree Manston might be a good option, but if you are a little west of The Wash when the problem occurs, Bedford is nearer. And with flaps likely to fall off, I think the nearer option is probably better.
I think I will keep the south of Blue One but, if the decided to go north of that airway, would it be Border Radar as suggested by ZOOKER?
For anyone interested in the flap track problem, here is a link to the NTSB report:-
https://www.ntsb.gov/safety/safety-r...88_131_133.pdf
Thank you all for your help.