I had a HV pop in a few weeks before the birth last time around, impatientlywaiting. I expect they'll do the same again this time around.
It's nothing to worry about, basically they'll just come in and have a chat for 20-30 minutes or so. Introducing themselves, handing over a load of leaflets and information guides from the NHS, etc. We didn't leave the lounge, so my tidying the entire house in preparation was unnecessary. 
In addition to that, she was just checking that I had some idea what I was doing, that I'd thought about where the baby was going to sleep and how I was going to feed it, that I knew the vaccination schedules, etc. Also making me aware of how to access help if I needed it for anything, and where some local groups for various things (baby massage, breast feeding, etc) met up and how to get in touch with them.
I seem to remember a casual question about whether everything was going ok with DH/family/friends as well, i.e. checking whether I had any abuse problems or similar. I just said there were no issues and we moved on. Better that they ask everyone than miss the one woman that needs help, I reckon. :)
You hear a lot of bad reports about HVs, but mine was lovely. Really grandmotherly and didn't care what choices I made as long as I felt I had the right information to reach my decision and be happy with it. She basically set herself up as a friendly point of contact if I felt I needed any help or guidance.
I rang her with questions a couple of times and saw her a few times after the birth. It was mostly just like having an experienced friend pop in to visit. She coo'd over the baby, checked everything was going ok, offered advice if I asked and didn't outstay her welcome!