Let's be clear: Clement repaid the personal expenses, claiming the corporate card was the only one on him at the time. All those times. But when MPs are losing their jobs over a cavalier attitude to expenses systems, it's no wonder the Mayor's furious.
In particular, Boris is hopping mad about business class upgrades for Clement and a colleague on two flights to China for the Olympics. GLA guidelines do allow business class travel for journeys of more than 8 hours, but the Mayor wanted to slash costs on the Beijing visits and set an example by flying economy himself. Apparently Clement missed, or ignored, that memo.
Despite his anger, Boris declined Clement's offer to resign but Assembly members are urging him to go. You can perhaps understand the Mayor's reticence: to mangle a phrase, to lose one Deputy Mayor may be regarded as a misfortune; to lose two and a senior advisor looks like carelessness.