Thanks for your response.
Mum’s actual annual usage based on the last 2.5 years s :
On peak 24 hr : 6550kWh
Off peak 11 hr: 6229kWh (2 large Storage Radiators, which are used 8 months each year.
Probably (?) because its a largish, 3 bed, 3 living room, 1950 dormer bungalow with cavity wall insulation, full-double glazing and loft insulation 100 /120mm loft insulation.