no demand from external stat,
with primary flow switch disconnected(red wires) boiler does not light, although pump runs and primary spindle operates switch, no dhw either.
with p/flow switch connected and plunger held down manually, dhw works,
i.e. turn on tap and boiler lights.
I expect that you have misquoted that.
Are you suggesting that it works properly if you manually hold down the pin?
How many contacts has the microswitch?
If only two then there is no possibility of a mis connection.
If three then it could have been connected tot he wrong terminals.
However, I expect this is a red herring and you have said something you did not mean.
Tony