DJ Taylor: Dutiful citizen Sally Layte's tasks are sadly nearly all superfluous

"Now girls," brisk and beady-eyed Sally Layte instructs her twin daughters shortly before their departure for school, "I'll be running the tiniest bit behind schedule this afternoon, so you'll have to wait in the library for a moment."

Eliza, the more good-natured of the two, accepts this information with a practised smile, but Marianne, who has good reasons for wanting to be home on time and knows that "a moment" means a good half-hour, if not longer, is disposed to be cautious. "Well dear," Mrs Layte replies, having attended to some remarks about other girls' mothers managing to be at the school by 4.15pm, "you know it's my afternoon for visiting Mrs Woodhouse."

"But why do you have to visit Mrs Woodhouse?" Marianne wants to know, at which point Mrs Layte's manner turns a little sharp. "Now dear, you know Mrs Woodhouse is becoming rather an old lady. And she isn't in the best of health. If I didn't go and see her, I don't suppose anyone else would."

In fact, Mrs Woodhouse is a sprightly widow in her early sixties with a brood of grandchildren permanently quartered on the premises, to whom Mrs Layte's periodic visits – always bringing a jar of marmalade to add to the two dozen others groaning on her friend's shelf – are something of a mystery.

Until her husband's salary increased to the point where she felt able to retire, Sally taught. Five years later, her life is busier than ever – a ceaseless dash across the part of north-west Surrey in which the Laytes reside, a permanent struggle to pour quarts into pint pots. An hour chez Woodhouse will quite likely be followed by a trip to the dump, some highly erratic gardening and – another of Mrs Layte's daily obligations – the walking of a neighbour's dog.

The curious thing about these tasks is that they are nearly all superfluous. The Laytes already have a gardener. The neighbour is quite capable of walking the dog herself. Even more curious is that these interventions frequently impinge on her own family's wellbeing: there is never any milk in the fridge and the most common instruction at suppertime, when Mrs Layte rolls up exhausted from some demanding afternoon of semi-public service, is that everyone should "go and forage".

On the other hand, as her husband and children know to their cost, it is impossible to rebuke a woman who believes, quite seriously, that she is doing her duty.