Cheers Malcolm, had a look at the Frost site. It's amazing what you can in a tin isn't it! In the end I solved the problem as when I got up in the morning the last test piece i'd sprayed had turned a fairly nice satin sheen, not quite as dull as the orginal finish but since I'd already paid for the paint I went for it.
Just been down to check now and it's looking pretty good, looks a bit like an eggshell finish that you get with the oilbased household satins. I did a bit of investigation on t'internet about matting agents and satin paints: Basically they are all chalking agents of some descriptinon (fine talc is the most basic) and so when the paint film dries and shrinks back they allow the film so be disrupted on a microscopic scale. This means that the light reflection is scattered giving a matt effect. Anyway they tend not be as effective with 2k paints according to one source (somthing to do with drying speed). For reference I ended up using approx 350ml of matting agent in 2 litres of paint! Only downside of satins is they don't tend to be as impervious as a gloss finish.