Worcester Boiler Engineers have been and fixed the problem.
A poorly fitted flue with a creased seal was letting 'dirty' air into the boiler.
They also reconfigured the Gas air ratio.
We have a 9 year old Worcester Greenstar 27HE combi which have had repositioned in our house from a bathroom location to a new utilitiy room we had built as part of a small extension.
When we had it moved we went from a flue through the roof to a through wall flue and a new flue was installed...