Thanks @Locky. The issue that I have with most natural fibre insulation is that it is not hydrophobic. This is really important to me because I live on a steel boat and any water absorbed by the insulation will rust the hull from the inside out! This is something that the insulation board does very well. You can literally soak Celotex in water for years and it doesn’t absorb one drop. I have actually fished old Celotex boards out of the river and used them for insulation and they are completely fine 
Thanks for the link though, really useful. It seems like there are plenty of alternatives out there that would work in lots of cases.