I don't know, Paul, as I have yet to have the pleasure of doing one. All I can say it that several posts indicate Valeo insist on Forte Stop Leak as part of the installation process for their warranty to be valid;
http://www.frenchcarforum.co.uk/forum/v ... ix#p371593" onclick="window.open(this.href);return false;
I found 25 posts under a search for 'Valeo Forte Matrix'.
EDIT; Maybe it is like putting Fernox into a new central heating system to prevent any lime scale from forming and causing future problems?