ok so the mystery deepens......
The good condition MC arrived, i fitted it and bleed the brakes every which way again....lever still not firm. Tried the fast flicking (missus got jealous
) but it didn't help. Had i just regretted buying a "good condition" MC cylinder and should i have just bitten the bullet and spent £150 on a new nissin MC?
Now here is the odd thing, whilst i was here this time it thought i would install the new discs i had lying around. Well, as soon as i installed, a quick bleed again and the lever was a lot firmer now and less travel?! WTF?
It still wasn't right though and as it was getting late i just clamped the lever back to the bar overnight. When i went to have a look in the morning once i released the lever it was still slightly firm (although still slightly better than before) but i now had a pool of brake fluid on the floor from the NS caliper. Took the caliper off and it seems the pressure overnight had forced it way past the new piston seal on one of the pots.
So it look like i may have pinched the seal when i rebuilt the caliper, but not sure why it didn't do this before, i'm assuming because it wasn't building up any pressure in the system.
So fingers crossed, if i remove the faulty piston, check the seals, re-grease and put it back together i am hoping this will resolve the issue.
So it would appear the a mix between new discs and a leaky seal (which didn't show up until the discs went on) was the solution