Hi Ian,
Glad to have you join us! Its always nice to have some more UK based TDHers!
Personally I always start from the bottom, and begin with the base colour of silver, others tend to apply their colours topically. In terms of paint, Hobbycraft is the best place to go, Bobamaker has put together a list of colurs for a helmet, but thats for an ESB. Try searching for the 'UK paints' thread.
I have used a humbrol acrylic spray for my helmet, which I think is quite accurate. I think I used light olive drab.
Good luck with your project!