'Heater' is underneath the plasticky bit between the bonnet and the windscreen. There will be a piece of plastic and a pollen filter on top of it.
When I say heater, I'm not sure if it takes heat from the engine, and it's a 'blower'.
If not, check the fuse for the heating element , and not the blower.
Another though, change the pollen filter, what with all this mighty cold weather, it's difficult to warm up when the pollen filter is blocked up. I think.
"C20 LET bang"