For a long (25mile each way) commute, what kind of bike should I look at? I'd really like to be able to commute to work. It's a long way though - 25 miles away. Too far for every day, but maybe I can cycle it once or twice a week.

I tried it on my old mountain bike a couple of times and it took me 2 hours each way, and I was very tired. I'm hoping a new bike might make it more do-able. I really can't decide what kind of bike to get though. My route has about 10 miles in the city, and 15 miles on country roads.

I'd like something reasonably light. Hybrids look a bit on the heavy side. Full on road-bikes look a bit too aggressive, and I don't think I'd like the drop handle-bars. Something like a more relaxed road-bike with flat bars would be good, but on digging through the web I'm having trouble finding anything that looks like that.

Can you recommend a suitable bike that would fit what I'm looking for?

I'm in the UK and could spend up to £1000 for the right thing.
I would defiantly not rush out and buy a new expensive bike.  Here are the steps you should follow: 

1) Try to make your bike faster.  There are  a few ways that you can do this but I am going to make two suggestions that will be cheap and easy.  


Get slick (smooth) tires.  The should put you back around $60 but should give you a good idea of what a road bike might feel like.  The difference in speed will be shocking to you.  
Put some bar ends on your bike these will help you to change your body position on your ride.  (More positions = greater happiness)




2) Before you buy a "new kind of bike" (ie: road bike, hybrid, etc) make sure that you try it.  I just bought an older road bike on craig's list for $20.  The bike was in very poor condition but it allowed me to figure out what riding a road bike is like.  I upgraded a few weeks later knowing what I really wanted.  

If you don't know what you want I would mess around with your bike a bit and then buy a couple throw-aways.  I bet that you could do that for $50 - $200 (not including resale revenue) but you will know what you want before you drop the big dollars.