Not sure how it is in the UK but here in the US with problems like these, I go to an automotive paint store and have them match me the color I need. Maybe a local body shop can help you as well...
I you're able to get help that way, have the store make you a small batch of color and make sure it's a single stage paint that needs to be catalyzed for durability.
You'll have to make a few passes in order to get the desired build up.
Good luck