Lots of automotive paint suppliers can mix any colour and put it in aerosol for you.I think some Halfords do this as well.You will have to pay 7 to 10 pounds though. Touch up pens /sticks are tricky (unless alfa supply them?)
PS. They are for sale on ebay
Pete, not sure if the latest European Products Directive is making it more difficult to obtain 'Touch Up' paints, restricting the use of organic solvents, but Autopaint and others have said they will continue supply until???? Yellow pages if you are struggling mate.