Electric starters. You could start up from batteries or using a GPU (as shown in my photo above). In later years the 'PEP team was limited to battery starts only as they didn't have a GPU available at the museum. I understand that only the Vanguard and the Belfast used electric starters on the Tynes, the other types all had air starters fitted. I looked back over some old posts about G-APEP and I found some links to an engine run on 31st October 2009.
The shutter speed for these videos led to the props appearing to turn really slowly, but I'm sure that they're at low ground idle at least. Also, the image looks a bit squashed horizontally.
This video from 2008 is better, but still doesn't show the correct rpm for the props:
Right, I should add these as well, the last landing of G-APEP at Brooklands (they ended up a bit short...) and the last time the Vanguard taxied on the Brooklands runway. As for stopping a Vanguard, it is worth pointing out that it landed on the northern half of the runway as Wellington Way had been constructed by then. Because of this, only about 60% of the original runway was available for her to land on.