I've always assumed that, like everything else in the series, the garden is a rented set. Remember a couple of series ago and the Routemaster bus that conveyed the fragrant one down streets that no Routemaster bus ever goes down? Come to think of it, there aren't any of those buses anymore, anyway, on a regular basis!
It's pretty tricksy when they film NL going into a house through a big Mayfair-type door, and then the next shot is of 'her' kitchen - ie the film set in Battersea. Why should anyone believe anything she says when the whole premise is a fake?
According to that article in the Evening Standard, the marital £33m flat contains 2 kitchens. Maybe Mr Saatchi uses one to boil his eggs, while his missus whips up gallons of minted pea purée in the other....