Probably the flame failure thermocouple has failed.
I did a quick search and found what appear to be at least 2 versions of the manual for this.
Page 18 of this one looks useful
http://www.acwilgar.co.uk/Boiler-Manual-PDF/Baxi/BAXI%20MAIN%20MEDWAY%20SUPER%20GCNo.52-476-39.pdf
This is the one from the Main website:, which isn't so forthcoming about the thermocouple:
http://www.mainheating.co.uk/docs/101209_Main_MULTIPOINT_BF_Guide.pdf
XXX, November 2016
