The upstairs will almost always be warmer since the heat from throughout the house is rising to the second floor. That being said, it also sounds like you could use some more insulation in the attic. I don't know about the UK but around here you can get insulation at a home improvement store (Home Depot, Lowes) and install it yourself. Rolls of insulation and spray in insulation (newspaper fiber - not foam) are both fairly easy. Often you use unfaced insulation batts in the attic but that may be regional.
A lot of people around here install whole house fans. They are basically a fan that installs in the ceiling that sucks the hot air from your house and dumps it into the attic. You crack open your windows so that the cool air can flow in (if you don't, you can bring in carbon dioxide from your heater). You can run them all day if the temperature is nice. We tend to run ours when it cools off at night and in the morning since our summers can be hot.
Some also install attic exhaust fans. These either suck hot air from the attic and dump it outside or force cool air from outside into the attic.
Both attic exhaust fans and whole house fans require the attic to be ventilated. Around here, attics often have ridge, soffit, or gable vents.
http://en.wikipedia.org/wiki/Whole_house_fan
http://www.popularmechanics.com/how_...c/1275656.html
http://www.thisoldhouse.com/toh/vide...047003,00.html