Mick, the secret is adequate exposure, Ilford's recommendation is "30-60 seconds at 46cm/18in from a 100-watt tungsten lamp" but go on to say 2-4 times this is safe. In the past I've also used a 275w Photoflood but the distance is increased to about 6ft (2 metres). So there's plenty of le-way.

Heavy reversal exposure particularly to sunlight can cause major problems with the image partially inverting giving a sort of solarised effect.