Thanks Daniel. In this instance, I used some 'plastic' type filler which I had left over from filling a dent in the lounge wall (I accidentally smacked a hammer in the wrong place!!). Normally though, for small bits like this, I'd just use a papier mache mix - flour paste and newspaper strips.
If you're contemplating anything similar, then avoid plaster of Paris for small areas, as it's liable to crack. There are modelling products that use the latter, embedded in a gauze strip, which can work quite well, but tend to add quite a bit of weight to the base. The 'plastic' type fillers are lighter and more flexible, thereby avoiding the cracking or pitting problem. One of the best, available in various sized tubs, is called 'Tetrion' in the U.K., but the one I had 'in stock' was 'Polyfilla', as shown in the picture here:-