Right so since last post I have replaced clutch hose & pipe and put in a new slave push rod. The new push rod was actually ~10mm shorter than the one that was in there.
With the new push rod installed (no return spring) I cannot get it into gear with the engine running, with the longer (old) push rod I can get into gear (still no return spring).
As soon as the return spring goes back on, I cannot get it into gear, with either push rod.

At this stage I think I will replace the slave cylinder (everything else around it is new, may as well!) although it does look to be working correctly with no leaks etc.

Does the slave cylinder have a seal spreader in it? Most new ones don't, but need it.
See my other thread re this. I make new ones...!

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Thought I'd post a quick update to happily close out this thread.

I replaced the slave cylinder this evening with a new genuine one and this has resolved the issue - with the spring back on i can change gears with the engine running. Took her for a drive around the block and the clutch pedal felt great!

Thanks to all who contributed advice!

Awesome - happy Mini =

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Videos - Turbocharged A-Series on Dyno & at Wakefield
Mafs waz not my strong subgeckt at skkol