Yup... just an update (because I noticed this thread comes up on google!)
It was the alternator.
I bought a replacement 150amp Denso alternator from a local motor factors for £96.60+Vat + £15 surcharge. Tried to fit it myself but even getting the belt off was a disaster - meanwhile by this stage I'd done 60+ miles without an alternator.
Then yesterday I rung the main ford dealer and asked if they'd fit it for me (expecting them to want to source the part etc), but they agreed, did it within the day, and charged £81 all in to fit it.
On the way to the dealership, power steering and ABS failed, but amazing, despite the voltage dropping to nearly 10v, engine-wise it didn't miss a beat. Dash was lit up like a christmas tree playing all sorts of tunes but it didn't stop!
The problem has gone away, and the dealership left the old alternator in the box for me to claim my £15 back.
Just under £200 down, but problem solved quickly. Really happy with the ford dealership for sorting it quickly, and don't get arsey about getting the bits themselves etc.