when i say "hand held" i mean portable rather than fixed to a wall, not that i literally hold it in my hands except briefly when turning it up or down. we have a plumber coming to look at it next week but thanks to all for the replies.
i have a SALUS RT500RF hand held controller and a VAILLANT combi-boiler. i recently had to change the batteries in the hand held unit and since then it doesn't actually control the boiler. both red and green lights are on on the receiver unit but even if the flame is showing on the controller...