As you can guess from the title, I have a Y-Plan central heating system.
The 3 way valve controller has a lever on it and there appears to be two
positions for it to be in. Left, marked as Auto and right, marked as Manual.
For some reason (probably by mistake by the previous owners) the lever
has always been at a position midway between Auto and Manual.

Dumb question I know, but what is the difference between Auto & Manual?
When I experimentally move the lever to Manual it feels like there is
some internal gearing that is being moved as there is slight resistance
to motion.

The only instructions that I can find on the web are how to install
Y-Plan systems. What I want to know is how to operate the system and why
I should want Manual instead of Auto and vice-versa.

Incidentally, I have noticed that when the hot water comes on but not
the central heating, some of the radiators that are close to the hot
water tank and the central heating control gear, heat up. Could this be
a symptom of the (presumed) incorrect setting?

Thanks.