Granted1 -
An Acrylic Based paint is your best option, airbrush applied, and then wet sanded and polished out to get rid of any overspray.
Sometimes paints and plastics don't get along, too. Some paints, when applied to some plastics, can cause a color change reaction. If there is release agents on the plastic, that can cause a color shift as well. Try to find a paint that is acrylic based, a good for use on plastics. Polyurethane based are very good as well. As always, make sure the area you are going to paint is clean, lint free, and dry! Use Ammonia or paint thinner to clean the area before painting.
Here's a couple options:
1 - Paint the WHOLE thing. That is the only way to insure the invisibility you are seeking. Be sure to prime the part first, and then apply you coats of white. Primer will seal the surface... and yes, they do sell primer in white! Its great when you have to apply white, or a bright color, like a red or blue to an object. White tends to be slightly translucent, so you will have to put on more than two coats. If you use a gray primer, you'll know when you have enough coats on when the white paint no longer has a "blue" tint. When I painted my Rubies Helmet, I used an acrylic lacquer primer, and Krylon Fusion. I am not sure if that is available to you in the UK
2 - Can you refit the part and shorten the gap that way?
3 - Would a little weathering cover up the trouble spot?
Are you applying the paint with an airbrush/spray can or by brush? Air brush is the best, and with patience and skill, you can achieve excellent results with a spray can. Brushes can pool up paint, which can "pull-up" ABS particles into the paint, and cause a discoloration of the paint, and surround plastic.
Check out the thread Acrylikhan's Helmet Progress as I have cover some of the topics here, especially masking off areas that you do not want painted.
If you have more questions, let me know... it may take me a day or two to answer.