We're slowly getting to grips with stacking Molly, but our latest problem is how to get her to move without looking up at us; this bad habit is making her crab horribly. If I hold a titbit directly in front of her she jumps for it and won't trot; anything but a loose lead makes her struggle and bite it ...

Try using a target stick, found it worked well to correct the same problem.

What's she like without any food if you are on the move?
Is it the lead she goes for? Maybe coat the lead in something like Vicks vapourrub so she won't want it..?

Would recommend trying to move her without food. I've also trained one of mine to respond to the command "head down" which basically means that I've taught her not to hold her head up so high or look at me when she moves. Unless a dog is really reluctant to move, I almost never move dogs with food in my hand because it encourages them to look at where ever the food is, which is usually not where you want them to look.

> We're slowly getting to grips with stacking Molly, but our latest problem is how to get her to move without looking up at us; this bad habit is making her crab horribly.

Try moving her without looking at her. One of mine sounds exactly like yours. If I look at her when we move she crabs, bounces and moves terribly. She likes to make eye contact with me.  I have found if I look straight ahead, concentrate on where we are gong she stops trying to look at me and starts moving straight. It is surprisingly difficult to not look at her as I want to see how she is moving but as soon as I glance down the crabbing starts again, If you can get other people to watch and let you know when she is moving well it will help.

I have found the same if you look at them they look at you.  Getting slightly ahead of them when you start also helps.  I'm not a great handler just adequate.