yes you should be able to.
you have a 2 way lighting circuit so you need a 2way dimmer switch like Pete says. TLC have quite a few handy tech links for various products so worth a look around their site (and theyre one of the cheaper places to buy electrical goodies too!). heres another..
http://www.google.co.uk/url?sa=t&rct=j& ... y6l2PMZ7Hg
look at your existing switch, there will be 3 connections, probably labelled L1, L2, and Common. a 2 way dimmer switch should be the same. note which wires (not which colour!) go into which terminals and do the same on the replacement dimmer. note they may NOT physically be in the same orientation.
there will possibly be wires of the same colour doing different jobs in a 2 way circuit - do not mix them up!
I AM NOT A QUALIFIED ELECTRICIAN! usual disclaimers etc.
and I would shut the power off completely when you do it ( as opposed to pulling a fuse or circuit breaker). You never know whos done what in the past!