Halfords don't seem to have any ready-mixed Alfa Romeo colours, whichever branch you go to. I don't know why. I had to go to my local Alfa dealer for an Alfa Red touch-up paint (couple of quid) - you'll probably have to do the same.
Halfords have a colour matching service in some stores now where they will make up any colour in touch up bottles and sprays. They even so a really fine needle nose thing for stone chips.
Maybe try a bigger Halfords store with a parts desk.
Have you thought about getting a Chips away guy to fix it? It would probably cost you £50 but you`ll end up with a better result than if you try it yourself.Especially if your cackhanded like me! wink Chips away home page