Most flood-lights are of the type show here, using a quartz bulb.
http://www.screwfix.c...150W-Black-Floodlight
I would suggest you remove the bulb and take it to any hardware store for a replacement (e.g Wilkingsons should stock them).
The important parameters are the size of the bulb (length) and the rating (Wattage).
When replacing the bulb, take care not to touch the glass envelope of the bulb – the grease from your fingers will cause a hot spot, dramatically reducing the life of the bulb.