I may only be a 'lay person' (Mr Glazier's words), and I don't know much about microbore (hence me starting this post), but I picked up a few things about mid position valves along the way.
Kirkgas, here in simple lay persons terms are the workings of a mid pos. valve;
Room stat calls for heat - paddle moves to open port A (heating)
Cylinder stat calls for heat - paddle moves to open port B (HW cylinder)
Room & Cylinder call for heat - valve opens ports A & B (ie. mid position)
Hope this helps. There are 3 port 'W' plan valves, but that's another story!
OP