yes peter bellamy was very good and he was of course a great harmony singer with harmony group young tradition.
tootler;parallel thirds,are accepted under certain conditions in classical music,they are rarely used for more than a few bars at at a time,and then when they are,itsOFTEN in conjunction with two other voices providing contrary motion.
If you were to take an eight bar folk song it would be pretty boring and unimaginative to harmonise it[for two voices] in parallel thirds,all the way through.[that is what I am talking about].
now when I am accompanying a song on THE ENGLISH CONCERTINA,I try and find a harmony line for an eight or twelve bar song,THAT IS INTERESTING,Not just parallel thirds all the way through,particuarly as my arrangements[to prevent the accompaniment from being overpowering]often consist of two voices with perhaps a third voice at a long note or cadence point.That doesnt mean that I never use parallel thirds,But I WOULDNT advise using parallel thirds all the way through a song[I hopethathas clarified my views]