Hi Neil, if your going to do a lot of this kind of photography don't buy rubbish or you will not be happy with the results, if your just experimenting and will be happy with so so results then go for the buget end.
I have the Lee system that fits to the front of the lens and you slide square filters into the housing, its a very flexible system but more expensive than one that you simply screw to the lens front
I have had great service from Warehouse Express and the prices are reasonable compared to to other suppliers
do a search for filters in the techniques section, I found this one but have not looked at it, it does say " everything you need to know about filters"
http://beta.photoradar.com/technique...-about-filters
Bob