Thought I'd update this to help others searching the forums.
The problem was the master cylinder failing. The lack of fluid coming out of the slave bleed nipple was the major indicator it seems.
A couple of points... first, it seems most parts resellers only have the left-hand drive part listed on their databases! The left-hand drive part has a ball joint, whereas the right-hand drive is a bush and pin. Took me three attempts to get the right part.
Secondly, it's a right pig to replace. The two nuts holding the cylinder inside the footwell are tricky, especially the right one (when you're upside down in the footwell) - I needed a thinner socket to remove it. A standard socket wouldn't go on. Knuckle joint and extender bar is also needed.
I also had to partially disconnect the fuel filter to disconnect the smaller pipe at the back of the egine. This part was particularly tricky without the right tool for the job.
Fitting the new part was quite easy compared to removin. The hardest part was poking the pipe on the new cylinder back through the bulkhead. I replaced the "clic-r" connectors with jubilee clips, although I'll be keeping an eye on them.
Hope this info helps someone somtime