WMG / Quatermass
The Quatermass serials take place in the same universe as Doctor Who.
How many universes can there be where 1950s-1970s England repeatedly attracts hideous alien villains vulnerable only to intelligent, eloquent scientists? Plus, the British Rocket Group canonically exists in Doctor Who (it gets passing mentions in both "Remembrance of the Daleks" and "The Christmas Invasion") and in 1963 it had an employee named Bernard (according to "Remembrance of the Daleks") who was apparently the kind of man you would "wish was here" when faced with complicated alien plots.
Somewhat inevitable, given the above. If he does exist in the Whoniverse, then regeneration would be the obvious explanation for why he has a different appearance in every story.
The aliens in the fourth Quatermass series were Reapers.
In this series, it's speculated that Ringstone Round and other neolithic circles are places where previous reapings occurred - and yes, the term "reaping" is used.
It's also speculated that the aliens are using genetic material from the abducted!