Thanks Scuzi, that's just the sort of info that helps.
FL90 is the notified minimum overflying level in the London TMA, however, on the day if things aren't too tricky we may say yes to lower.
Understood. Technically, I'm not overflying the TMA but departing from or arriving at a TMA group airport that shares its STARs with Stansted. But perhaps that makes no difference, as the procedures all assume that arrivals will start well above FL90.