The bit the key ring goes round on flip key on my Mk4 has broken, ordered a new blank key off fleabay. Is there a time out when I swap the parts across, I don’t need to replace the full key just the bit that has the switches on? I seem to remember years ago if you didn’t swap it all across in something mental like 10 seconds it wouldn’t work?

Is there a reprogramming sequence as well? 

A lot of people online seem to think you only have a certain amount of time once you remove the fob battery. Of all the keys I've ever changed batteries on, I've never experienced them losing their memory (up to half an hour sometimes).

Ok, I’ll also need a spare key cutting, years ago you could go down timpsons and get one cut. I don’t need a remote one just a basic one to start the car and open it if I lost the main key. Is this a Ford only job? 

No. Ford dealerships don't cut their own keys anyway. A local auto-locksmith will be able to provide, cut and program a new key for you.

You might as well get a remote key. A standard one won't be much cheaper to buy, and the majority of the cost will be for labour/tools for cutting and programming the immobiliser.