What gears should I go for? I have a Trek road bike (although you might call it a hybrid since it has straight handlebars. I do about 3,000 miles per year on it for general health reasons (up to 30 miles at a time and averaging 13 mph) and I'm not into racing, cycling clubs etc (I'm coming up to 70). I've just replaced the front chain wheel with an identical version (Shimano Acera FC-M361 48-38-28T) and also the chain. I need to replace the rear chain wheel but before I do I thought I might ask for some advice. 

The supplied rear chain wheel is a Shimano HG-51 8aw (i.e. 11-32T). Virtually all of my riding is on levelish ground and UK roads (as bumpy as hell!) with pretty shallow inclines - no off road riding. I find I rarely use the high and low front gears and virtually never use the bottom two rear gears. So what would be your advice on what gearing I should buy for the rear chain wheel replacement, and if you feel so inclined could you comment on the overall front and rear gearing - I feel its probably not optimal since I only seem to use 5 or so of the 24 gears!
If you aren't using many of the gears, then you should definitely go for something with a smaller range. The triple crank will give you plenty of range even without a wide range cassette. They sell 12-25 and 12-23 cassettes which may be closer to what you are looking for. I have an 8 speed cassette with a 52-42-30 crankset and I opted for the 12-23 cassette. I find there is plenty of range for climbing hills. The nice thing is that the gears are so much closer and I find that it's a lot easier to find the right gear in order to maintain an optimal cadence.