The http://www.est.org.uk/bestpractice/ site contains some useful pointers indicating that this is something that can be done and is sensible to do.
They suggest either a rigid insulation (e.g. extruded polystyrene) supported by nails or battens in the joists, or 'loft insulation' supported by netting stapled into the joists.
However, they caution letting the polystyrene come in contact with electrical cables which can go brittle, so that restricts that suggestion, and they say that any insulation needs to be kept in contact with the floor above, so the idea of suspending loft insulation by netting seems practically impossible.
Hence the question - has anyone actually done this? What are the practical solutions?