In reply to James Malloch:
I've carpeted the floor on ours. Done cheaply so it can be replaced easily.
wrap insulation from b&q for Windows. Made into blinds. Curtains over the top.
Thick curtain for cab access.
Block vents (Not roof vents) in winter with insulation.
With the gas heater we had 28 degrees after 2 hours. Outside it was less the 5 degrees.
Edit: charity shops are your friend
Don't Be scared to do it on the cheap. It does the job we wanted it to anyway!
Post edited at 14:43