Some people I know went with a big commercial like Cottages4u which require a big commission but then you get lots of people booking you and can then work off return visits if you go it alone after say a year (plus word of mouth from them). But another person I know uses Holiday Lettings (think it's called) or there's Owners Direct (name again?) and they just charge and couple of hundered pounds or so for the year and that's it, so you have to weigh it up.
We have locally a Business Gateway setup whereby you can do lots of courses for free - marketing, tax liabilities, how to set up your website, google search engine ranking, facebooking, all sorts of stuff, and the person running it keeps in touch with you as a helper, you can even get free one to one help specific to you. The lady running the courses I was on even copied lots of info on holiday letting which I'd copy for you if I knew where it is - there's probably something similar on line, things you should know etc, lots of things I didn't even think of. The courses were extremely professional, lunch thrown in for all day courses etc, I was very impressed. May be a similar thing to help you in your area. All the best!