Margaret John appeared in two 'Doctor Who' stories - one with Patrick Troughton as the Second Doctor, and the other was "The Idiot's Lantern" with the Tenth Doctor played by David Tennant. As far as Toobworld is concerned, she holds the record for the longest span of time between appearances in 'Doctor Who'. (But if you count the Peter Cushing movies as canon - which I don't - then it's Bernard Cribbins who holds the record.)
Her presence hasn't left us yet in the TV Universe, as Ms. John will be seen in the upcoming HBO fantasy series 'Game Of Thrones', which I'm eagerly anticipating.
But it was her 'Gavin And Stacey' role as Doris, the randy old neighbor on the island of Barry in Wales, for which I'll remember her best. If you've never seen the show - well, first off, you're missing something special! - Doris was something akin to Mother Dexter on 'Phyllis' thirty years ago. Tart-tongued, potty-mouthed, sexually active, and you're never quite sure if she was telling you the truth about her latest exploits or if she was just slinging the bleep.
Here's an example of Margaret John as Doris...
Efallai y nos da a Dduw bendithia