Thanks Bobpalt I will have a look today, hoping it is there.
Spidey, it is a valve that automatically opens when the temperature reaches app. 4 c so that the water does not stay in the boiler and freeze.
There are different types depending on the boiler
Electrical safety/drain valve (red button) as on page 6 Here.
Frost control (blue button) as shown Broken Link Removed.
Manual (yellow) lever as shown in the attached diagram.
Thanks GJH that's excellent info, ours is the manual yellow lever valve.
This forum has a lot of knowledge within its members, I am not a forum person but will certainly stay with this one, thanks again.