Dave,
Spot on with the diagnosis ! The reed switch built in to the plastic hinge shroud on the freezer compartment door was not making 100{e5d1b7155a01ef1f3b9c9968eaba33524ee81600d00d4be2b4d93ac2e58cec2d} of the time – I think the shroud had moved / got knocked etc and the distance between the switch and magnet was too great.
A slight adjustment has cured this and now all is well. Of course, the circulation fan switches off when the door is open – this didn’t even enter my head until you mentioned the switches and obviously explains the random door alarm sounding.
Many thanks for taking the time to respond and fixing my issue.
Rob