You could try & phone the manufacturer, or even your gas supplier. It goes without saying (yet I'm saying it anyway) you don't want to mess too much with your boiler :)
I had a problem with my boiler (pressure too high) & I contacted my gas supplier, who gave me the boiler manufacturer's customer service number, and was able (touch wood) to solve the problem over the phone.
Vallient's website: http://www.vaillant.co.uk/Home/
Before writing this answer, I did a quick search and the following thread might shed some light on the subject.
http://www.ureader.co.uk/message/1575363.aspx
If you do have success, I would urge you to post back the ultimate answer here so that others can benefit.
Chris Murphy, September 2006