Why does it take so long for supervisors to get back to enquiries about PhDs? I know they're busy with their own research but it doesn't take that long to write an email!

I know what you mean! It'd just be nice to know, even if it is a rejection. At least you can forget about that one then. It's better to know than wonder waht's happening! Like you said, it costs nothing & doesn't take long to write an email, or even forward a pre writted one!

P.S. Excuse the typos, I'm trying to watch the rugby at the same time!!

i went through that last year. They are just lazy b******s and dont get any better once you are working for one either.

It is true that many supervisors are very busy and it is also probably true that some are just being lazy or rude. You many never know which. If you haven't received a reply, write to the name listed in 'Enquiries To' in the departmental information, and explain that you're waiting for a reply from the supervisor. These names are often secretarial staff and are generaly better organised.
Departments and supervisors are as keen to recruit good PhD students as you are to find a good PhD place. If they mess you about, look elsewhere.