Tex: Difficult to see, even blown up, but have you got one of these between the boiler ports hot and cold? It's a mechanical bypass [vanne melangeuse suedeoise- never seen one in the UK], with a dial indicating the amount of boiler hot to cold bleed-back. They come in 3 port [voie] versions too. They do tend to gum up solid, so adjusting the bypass is often impossible after a few years.
http://www.ebaneo.com/gamme-1820-vanne-melangeuse-4-voies.php 
