Are you sure? I want a static IP - I would be happy to pay a good £5 a month for it, but I just don't think they give them out. What could be a possible reason? You are not supposed to run a server under the T&C's...
Gadge, I already mentioned DynDns. Sing up for an account there and then input the details into your router and this will automatically update your IP with them, allowing you to use a hostname which you will set in their domain.
I haven't got a copy of Sky's Ts & Cs, but there are any number of reasons you might want a static IP. Remote assistance for/from family or friends, ssh and/or telnet access to your box, access to your files from outside the home (uni, work, whatever)... The list goes on.
Whilst Sky say you're not supposed to run servers, in practice I've never had anything said about it from UK Online even when I've mentioned running them. I think they're just covering their backsides in case of someone abusing port 25 for spam, for example. Despite what the Ts & Cs (and some sales reps) tell you, the ports aren't even blocked.