***UPDATE***
Went and got a power bleeder yesterday, tried it from the top 2-3 times, no joy! So as said on here, tried from the bottom up (reverse bleed) still, no joy.
It's definitely sucking air in somewhere but I cant for the life of me tell and there's no fluid leaks either.
So what next? I really
really don't want to have to take the box off again but it's starting to look like I'll have to. I don't know if it has any significance but, the master cylinder holds very good pressure with the pedal up. It held 10, 15 and even 20 psi. I know it has to be a seal but which one?
There's, from top to bottom...
Flex pipe from the master cylinder to clutch master cylinder Jubilee clip at the top and one of those VX ones at the other end.
A metal pipe from clutch M/C to flex pipe which have 2x seals in each end and are only held in by clips that look like (n) that.
The flex pipe goes in to the box to a pipe on the slave.
The slave cylinder did not appear to have any leaks and appeared in good order...
Where to start?