As always, incredibly helpful feedback and suggestions.
I went with this stuff
uxcell car noise sounded deadener insulation mat "- looks about the right thickness and it's water proof and heat resistant and looks like it's easy to cut to shape. I'll let you know how it works out.
I'll also try to set fire to the existing material, once I've removed it from the car, to prove, without doubt, that it's not asbestos
