Literally just ordered yesterday!

There were (and still are) some decidedly crappy heater valves available.  Pick your parts and supplier wisely.
 

Mini Spares retooled the orginal design in 2014 and they work superbly - https://www.minimania.com/part/ADU9102/Classic-Austin-Mini-Heater-Control-Valve
I installed and tested the prototype daily on Betty for 30 months before I finally had a self-inflicted problem with it.

There was a batch of bad heater valves out there for a while, just replace it from a good supplier.
Hard to say what caused the driveshaft to disconnect itself whether it was a bad part or you did not get it completely popped in place. I would just replace the circlip and refit it and see what happens.

Well, something is in the air, but my almost brand new heater valve (Mk3/4) is leaking at the brass circular plate (where the plunger arm goes in) and my left outer drive shaft removed itself from the hub and slide-lined me until mr Tow Truck showed up.

I continue to be plagued with these new (quality?) parts that continue to fail quickly - has anyone seen their new heater value go bad within a year and leak?

And anyone have any ideas how the left outer drive shaft may have popped out from the outer hub?  I assume the cir-clip may not have sat or locked into place correctly.  Those are all new parts as well (new hubs, etc....not drive shaft).  I am using my old 998 drive shafts on a new 1275 engine and assume the tolerance is fine here - but tell me if I should be concerned.

Thanks for any feedback you can provide.