As I suspected, the heating saga did indeed continue. We were disappointed when the venting continued but at least there was only one other valve that could be causing the problem. I failed to get a replacement locally but sourced one on the internet which arrived the day after it was ordered. Pretty impressive. It went in pretty straightforwardly and now things seem to be back to normal. Just in time for the temperatures to dip and for the fire to be needed in the evenings.