Time Lords could only regenerate 12 times - until now, it seems, when we have Matt Smith starring as the Eleventh Doctor. The rule, established and repeated over 25 years of the show, now appears to have been revoked with an aside in the spin-off show The Sarah Jane Adventures.

In a two-part story called The Death of the Doctor, in response to Clyde's (Daniel Anthony) questions about regeneration, the Doctor gives a far higher figure for the number of times he can change than the Time Lords' statutory 12.

This brief comment reverses what's been a staple of the series since The Deadly Assassin, a 1976 serial when Tom Baker was the Fourth Doctor

But go further back to The War Games in 1969 and the then Second Doctor Patrick Troughton said that Time Lords could live for ever, "barring accidents".

The limit was also dodged in the case of rival Time Lord the Master, who used up his regenerations but was given a new "life cycle". And while it won't say much, the official BBC Doctor Who website does suggest that the demise of the Time Lords as a species and a society in the Time War might somehow take away the 12 regenerations limit.