Well I've done some experimenting and have come up with a solution. I think this is one of those temperamental things that depends on whether the eggs are out of the fridge or on room temperature. Anyway, for anyone that's interested I used the butterfly with the egg yolks and sugar for 5 mins on 4, at 50 C. Then I added the cream and milk. These take a while to heat up straight from the fridge, I think it's more gradual than on a stove. I then went for 8 minutes on 90 speed 3 as suggested, and custard was not cooked. Kept going in 2 minute intervals, then 1 minute. It took 15 minutes to thicken to the right degree, and a perfect result I have to say. I think maybe if used on 80 C you would need to go for even longer, but there would be no risk of curdling. I am however going to use 80 C for Hollandaise, as I think this is even more temperamental. Thanks for all the helpful suggestions everyone!
