Firstly, where is your BT master socket? Do you have a socket downstairs instead of on the top floor? Due to how wireless works, you may find having the router downstairs works better and you can still receive a signal upstairs.
Assuming you don't have a socket downstairs, the best bet is to get a pair of homeplugs like TheSpook has linked to above. You can get some from ebuyer like these
What you do is plug one near your router with a cable. Then the other downstairs and a cable into your laptop.
However, if you havent already spotted the fundamental flaw in the plan, its that you will need a network cable connected to your laptop at all times. Not ideal if you are sat in your lounge and since you mention laptops (ie. plural), this could be a pain.
You can however, buy one of these which uses your power lines to extend the network but then wireless from the plug to your laptops. However, you will need a normal homeplug like this to plug in upstairs and connect using a network cable to your router.
Now in theory homeplugs all conform to the same standard but in practice they can be a bit picky so you may need a techie friend to sort it out for you but of course the costs are far higher than a pair of standard homeplugs.
It all depends on what you need and your budget.
Personally I'd drop a network cable down the side of the house, drill some holes and install another router.