I agree too - the Airfix kit is around 40 years old, with everything that means, plus the ageing of the moulds.
Regarding your canopy moulding for the B&V, it is rather a difficult shape to achieve by the method I described, compared to the Mossie, which was also much larger, in 1/32nd scale, and therefore easier. Using a plaster male mould can work on some 'solid' subjects, but is not ideal for clear moulding, due to achieving desired internal dimensions and smoothness of surface. A polished resin male would be better, but again, obtaining the dimensions would have to be spot on first time, whereas balsa can be sanded back to reduce overall size if needed.
The 'melted' plastic certainly has potential, but controlling the evacuation of air bubbles, or worse, bubbles caused by chemical reaction, may take some overcoming.
For the Grieff, I would go with the Falcon vac-form canopy - carving or producing an accurate male mould for a home-made canopy will be rather difficult.