Oberon slave cylinder - problems!

Hi all. Looking for a bit of inspiration here.

I was losing a little clutch fluid at the slave housing earlier this year. About a reservoir full every 300 miles.
I was due to go to the NW200 and didnt want any hassle so I decided to get an Oberon replacement.

It arrived eventually, 7 weeks! - long after the trip away. Buy anyway it arrived a couple of weeks ago so I've fitted it.
Bled and bled the bugger to death. No air showing, but its not fully disengaging the clutch.

To the point that if its in gear and you pull the clutch the drag at the rear wheel is very restrictive. So not set properly.
The instructions mention that the push rod measurement from the face of the crankcase is meant to be 37mm. I measured mine at 40mm.

Now I am wondering one of two senarios.....
Are my clutch plates worn 3mm beyond tolerance?
Or I read tonight on rsvrnet that people have had issues with the Oberon unit. Bigger piston + same displacement of fluid = less piston/pushrod travel

Help! MOT due Tuesday, and I'll end up needing a new clutch just getting there and back.

I have a copy of the service manual on CD, and there's no mention of the 37mm measurement. Where did Oberon get that from?