Aves Apoxie Sculpt. It's a two-part putty like Kneadatite/Green Stuff and the Milliputs. I use shaped toothpicks, a few rubber clay shapers, and a hobby knife bladed to work the putty with a final smoothing with a small wet paintbrush.Standard stuff, really. Working time is about an hour; full cure in four hours, but can be force cured with heat. It can be feathered, cut, and sanded to a smooth surface after curing. There are other colors - I prefer the medium gray, but this last batch I got was all the shop had and it's mixed to a white. I have no idea if it's available in Europe or the UK.
Glen