Hi jimblackler,
Welcome to the forums!

I'm sure ivan louette (our filters guru

) will have some excellent advice for you. Honestly I've found it frustrating to learn how to create filters, although I will try again soon. But I think it would be much easier to just draw the icons by hand. Basically you would draw the shape, duplicate it, move it slightly, add appropriate colors, and give a slight (very slight, to my eye) blur. Done!
But if you're bound and determined to use filters, it will be worth your wait for ivan
All best

Edit -- Oh wait! When I first followed your link, it opened the image just above the image that I think you want us to see. Because the 2nd time I opened it, it showed the finished version. So my instructions for doing it by hand above don't apply. However, I do still think it would be easier to draw it by hand. And now that I read more on the page you linked to, it appears to be instructions for drawing it. So now I'm thinking that you have some reason for not drawing it,, and using filters instead. So still, ivan will be your best bet!