When whoever did chose "How a Folk Song Should be Sung" for the title of the programme on 'The Critics Group' they immediately destroyed any chance of people understanding what the Group was about (sorry to bang on about them again, but as far as I know, they did fore work on 'the folk voice than anybody I know)
Nobody decided how a folk song should be sung and nobody ever should
Based on MacColl's theatre work we examined how the voice was produced and where it was possible to take it if you learned to use if it full and to iron out any long-term problems

Peter Bellamy isn't a bad place to start, Peter had a natural vibrato, as had MacColl, which got in the way of producing what people had described as "The clean, clear folk tone"
Both were aware of the problem; MacColl attempted to suppress it and largely succeeded, though it sometimes surfaced when he was over-tired - he once told be he couldn't bear to listen to his very early records, vibrato being one of the reasons
Peter may have tried to get rid of it but in the end he seemed to embrace it and it increased
The last time I spoke to him (of the few times I did) he described his singing as "Larry the Lamb impersonations" and appeared to be self-conscious about it
Though it sometimes turns up in source singing it is, I believe, rare - Fred Jordan being one of the great exceptions and, I think, his became more accentuated towards the end
There have always been arguments as to whether Joseph Taylor decorated his songs or whether he sang in vibrato

The argument MacColl initially put forward was simple; one of us really use or even know our natural voice once we become adults, the way we produce it is largely decided by outside influences (children are the nearest ones to use 'natural' voices 'naturally')
For instance, 'if you spend 8, 10, 12 hours a day in a noisy environment - a docker, miner factory worker (particularly a steel-worker), you adapted your voice to your needs to communicate, and that's the voice you took home
If you worked in quiet surroundings you controlled your voice to suit those conditions
I was an electrician and worked mostly in people's homes so I instinctively adapted the way I spoke to where I spent a great deal of my time

The argument was that our folk songs covered all aspects and ranges of human experience and emotion which demand different 'tones' - you don't use the same tone to chat up someone you fancy that you would if you were describing a football match - not if you wanted to get anywhere
What you do on each occasion is instinctively let the subject choose the way you speak about
Similarly, you don't use the same tone for a love song, a murder ballad, an erotic romp, a complaint about working conditions, a shanty....

The voice is like the brain - we use only a tiny fraction of its capacity and capability
If you want to use it to the full you have to understand how you produce it and learn how to expand it
MacColl introduced us to the exercises they used in Theatre Workshop and they worked to the extent that we practiced them (the same with his relaxation exercises)
This sounds time-consuming - it isn't
Once you learn these techniques they become almost instinctive - the song chooses the manner in which you sing it

They help with problems too
I stopped singing for years - when I stated up again I found my range had reduced; I could no loner sing Flying Cloud and Sheffield Apprentice - my rangiest songs
I worked at the former and gradually won it back, I found a better tune for Sheffield Apprentice anyway

These are techniques and should never be the main part of singing - that should ciome from interpreting and enjoying what the song means to you