After a chilly and breezy Friday night, both these readings had dropped by just 1 deg overnight and still no heating. During the day the readings dropped a further 1 deg (tilers in - patio door fully open!) then raised a little towards the late aftenoon/evening as the sun peeped out.
Saturday night was again cold (as snow moved into northern Britain) but temperatures only dropped a couple of degrees in the house overnight, only to rise again during today to as much as 23 deg. in the ground floor and 20 deg. in our bedroom due to solar gain, despite the continuing biting cold wind outside (and leaky patio doors!).
In essence the house was behaving passively, with perhaps the circulation pump (set to continuous within the heat pump unit) helping to spread the heat from solar heated rooms to the rest of the house, an effect which will be helped further by the MVHR unit when switched on.