Earlier this year, the Warwickshire and Northamptonshire Air Ambulance triumphantly paraded the Hollywood star Rob Lowe visiting its service. It was a superb coup that surely put many charities to shame for resorting to Z-list celebrities.
Even so, it was all rather odd. There was no explanation in any of the coverage I read of why Lowe, who was in the UK to promote his autobiography, was the charity's chosen celebrity or why it was his chosen cause.
It reminded me of a time when the actor George Takei, alias Mr Sulu from Star Trek, appeared in pantomime in Oxford. There's nothing wrong with Mr Sulu - on the contrary, he's famous, probably charming and, most importantly, usually available. But Mr Sulu in Oxford, in panto? Someone must have beamed him down in the wrong place.
I too have arranged some odd matches in my time. When I worked for a sporting charity, I persuaded the former hostage Terry Waite to be one of its patrons. He was wonderfully kind and helpful and had magnetic presence as a speaker, but some people still asked: "Why him?"
Sometimes the mismatch is apparent only with hindsight. For instance, I once persuaded the former England rugby captain Lawrence Dallaglio to appear on the cover of a sporting charity's magazine as a role model for young people. Unfortunately, this went to press shortly before a tabloid newspaper claimed he had admitted having taken recreational drugs. But never mind: I repaired the damage by persuading the Olympic champion sprinter Linford Christie to be on the next cover, which went to press shortly before reports emerged that he had failed a drugs test.
It's great when the match really works, though. For example, I think of Joanna Lumley recalling her father's career as an officer with the Gurkhas when she fronted their campaign for UK residence. Or the dance group Pan's People, regulars on Top of the Pops in the 1970s, reuniting 20 years later for a Save the Children photocall when I worked for the charity. Who's going to turn up for them, I wondered? How wrong I was. We were besieged by a tabloid scrum and the former dancers spoke movingly of their hopes, as mothers, for children around the world.
The potential for mismatches, however, is glorious. How about Arnold Schwarzenegger for the Family and Parenting Institute? Or Jeffrey Archer for the Institute for Global Ethics? And the comedian Jim Davidson might not be the first choice for charities in a wide range of fields, such as marriage guidance, disability or race relations.
Then again, a fallen celebrity's road to redemption could be superbly newsworthy for the right cause. So step forward, Ryan Giggs and Tiger Woods, and let's see you front the campaign for abstinence in sex education. Headlines like "Giggs Says Don't Score" almost write themselves.
Martin Edwards is chief executive of the children's hospice Julia's House