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!
I'm posting this under this subject because we are talking about cream.... Last night we needed some custard real quick (and the shops were shut). The only thing we had available was Hi-Lo (skinny) milk so I thought we'd use it as there shouldn't be "too much" of a difference being milk. Well, there is. It sort of curdled and looked like custard gone wrong (cos it was). Taste was there, but looks were funny. Was a good experiment though and we still ate it, but will never do this again. Hope this helps someone else in the future.
I was told the best cream for making butter is the liquid consistency one which comes in a mini milk carton - so that would be pure whipping cream I guess! I have to confess I haven't made butter yet........don't know why, just haven't summoned the energy to do it.