I'll sing fine one day and absolutely terrible the next. There seems to be no control or consistency

Been there - my sympathies. As far as singing is concerned, I think it's important to sing every day - and sing in the key(s) in which you would sing in front of a live audience.

It's all too easy to sing comfortably at home and then find, when you're singing out to an audience, that you need to be a tone up, for example. I also find that, if I don't sing every day - at a volume which will fill the street where I live (lord help the neighbours) - I might struggle a bit at the next live performance. The voice is an instrument and, like a violin which dies a little if not played regularly, needs regular exercise and use.

So - when you sing your Bert Jansch songs - pitch them at a level which will project to a live audience (even though Bert's a bit of a mumbler sometimes), and sing them every day.