That is kinda the approach I was thinking of. I knocked this up last night. As for getting metallicity when using mix rather than add - this can be avoided by attenuating the glossy using the fresnel input into the mix shader fac slot.
Obviously - the colour and SSS scale in my node setup may need tweaking if used in the OPs scene - but I’m quite pleased with this as a quick test.
Hey guys, I’m sorry it took soooo long for me to answer again, but I just spend the last 8 hours doing a high res render for print. 
So, as some of you said, when the client said “realistic”, they just wanted to be more like references and other 3D renders they’ve seen, so a physically and accurate, believable representation of the milkshake material was not a priority.
I’m not totally convinced or happy with the final result, but they approved it, and they will do a LOT of photoshop over the render at the ad agency anyway hehe.
@moony: my final shader looks a little bit like the one you last posted, but just with SSS at a pretty low scale.
Thank you very much for your help and your information guys; I learned a lot from the tips you gave me, but unfortunately in advertising most of the times you end up doing what the time allows and what the client wants (good or not). The next time I’ll have a better understanding so the final outcome will be better 
Here’s the approved image:
And the high res: https://drive.google.com/file/d/0B7ECfNWJoP-OZjIyR0FFV0Y1alU/edit?usp=sharing
Lighting looks a lot better - and as long as the client is happy - that’s all that counts 
I agree that bringing the strawberry texture in would add continuity, which would enhance the realism.
(edit) ah I see it is done…
