BEagle - cheers. Difficult to forget that jet.
spekesoftly - that tread pattern was not normal.
PN - I can't remember exactly, but it would have been about twenty minutes. Night scramble from Wattisham for suspect contact north of Heathrow. Told to maintain 5000ft and expedite, hence about 620 IAS.
Fortunately contact identified shortly afterwards so went home a bit slower and higher.