Joan Smith: We can work it out? Try singing that to Heather Mills McCartney

Did I blink or something? One minute Sir Paul McCartney was supposed to be comforting his estranged wife, Heather Mills, ringing to offer support during their separation and divorce, and the next he's getting cross about cleaning fluid. I mean, cleaning fluid? Little did I imagine all those years ago, when I heard him singing "We Can Work It Out", that he'd one day turn into a man who argues about Vim.

No amicable divorce for the McCartneys then, even though it might have been hoped that the former Beatle was both old and wise enough to stay friends with his second wife. It isn't easy to end a marriage on good terms but it certainly isn't impossible, with restraint on both sides. Any sensible person would advise them to avoid a repetition of the incident last week when Mills arrived at McCartney's home in St John's Wood with their young daughter and apparently found herself locked out.

Just about the first rule for separating couples is not to squabble in public, followed by an absolute interdiction on encouraging family and friends to take sides. In this instance, much of what's been written has focused on the financial terms, with extraordinary sums being bandied about; some commentators have suggested that Mills might be entitled to a quarter of McCartney's fortune, which would give her a settlement in the region of £200m.

At heart, though, I don't think that this failed marriage is so different from any other. The first thing divorcing couples lose is a sense of proportion, so that mildly irritating habits take on the complexion of insupportable affronts. What lies behind these otherwise inexplicable flare-ups is the working out of subterranean passions, not least a corroding sense of failure and disappointment.

Some of this is, I think, built into the institution of marriage through the unrealistic expectations it encourages; very few people consider, as they exchange vows in some suitably romantic venue, that there is quite a high chance that they will change their minds. Living with one person for the best part of your adult life isn't easy, especially at a time when we are all positively encouraged to develop and try new things into old age.

Some couples are able to acknowledge that they have grown apart and set about untangling their joint existence without too many recriminations. What this should tell us is that it isn't money that matters so much during a divorce as power: the precondition for an amicable settlement is a sense that neither party is losing face or being unfairly treated.

Clearly it was this issue that drove Diana, Princess of Wales, to fight dirty when her marriage went wrong, and it's probably one of the reasons why Heather Mills last week retained Diana's divorce lawyer to act on her behalf. Mills is a woman battling for survival, besieged by spiteful allegations in the red-tops and now, it appears, deserted by the powerful man she could reasonably have expected to protect her. It would be astonishing if she did not feel isolated and vulnerable, especially as it looks as though McCartney regrets marrying her and is punishing her for his own poor judgement.

This was never a marriage made in heaven, given the age gap and the relatively short time between the death of McCartney's first wife from cancer and his ardent courtship of Mills. He could and should treat his estranged wife graciously, in recognition of all the advantages of power, wealth and status he enjoys. If he does not, he can hardly complain that their separation has so swiftly descended to the point of divorce lawyers at dawn.