Spare Keys

I only have 1 complete key for my 156 - thankfully this works perfectly. I know I need to get a spare but at the moment I have a "partial" spare key.
Partial in that I can use it to unlock the car manually but can't use it to start the car. Took the fob apart in case it was something silly like no battery but there was nothig there at all! Do I still have to go down the whole new key route or is there a way to buy the missing parts and get it coded from there?

It sounds like you don't have an immobiliser chip or remote circuit board.
For a black key, the easiest option is to transfer the key blade to a donor key and have the car coded to accept it. This assumes that the donor key has a better case than yours, otherwise just take the internals and put them into yours.
For a red key you can take out the board and immobiliser from a donor and put them into your key

It's a black key and doesn't have anything inside. No circuit board or anything. All it has is the plastic housing with the key blade. The casing isn't in that great a nick either. Where would I get a new casing and circuit board etc?