Found out that the Blen's a pussycat as long as you take off from a longer runway with less wind!
I was persevering with the the stock cross country mission and still no go after a huge number of attempts.
Set up my own mission in the FMB flying from Biggin, with 4x250lb bombs on board and it was a cinch!
Oil at 40, heads just under 200, no carb heat, no boost, throttles at about 65%, mixture in the 'normal' range.
Doddle