The outdoor scenes were shot on location using 16mm film stock - the indoor scenes were mostly studio based and recorded on video. The early studio 3 tube plumbicon colour TV cameras couldn't cope with 'speculars' or naked flames, hence the "comet tailing". Later versions employed complicated circuitry that reduced significantly this unwanted effect, but when broadcast CCD colour cameras became available, the problem went away. But the change of quality between film and video edited together in the same episode could never be overcome.
I seem to remember that the Poldark House was in fact the farm at the top of the mine road leading down to the Crowns' section, Botallack, but a lot of artistic licence took place - parts of the same scene could be shot in several Cornish locations, on a tight lens, if the director thought it necessary. Keen eyed viewers still noticed, however.