Well I've learnt a bit this afternoon.
A local farmer came round and confirmed that the calf hadn't suckled. I had assumed that since I had seen it poo on the first day, it was bright on the next couple of days and wasn't mooing that it must have been suckling. How wrong I was. At least I now know what to look for to check if a calf is suckling.
We've now managed to get Mum in the crush and hand strip her and the calf has had a good feed. We tried to get the calf onto the teat and after being completely clueless to start with she did show small signs of getting the idea of sucking on the teat. Hopefully she'll be strong enough later to make a more determined effort in the field. I'll give her a bottle until I know she's got the idea (assuming I can still persuade Mum back in the crush again).
So, hopefully disaster averted, albeit I am concerned that she missed out on the initial colustrum. I'm just amazed that a calf can survive for 5 days with no sustenance.