Have a look at this site (I've used them loads of time and they are great), you can specify location, special needs (eg different access levels), pet friendly etc etc. They are much cheaper than the organised cottage holiday companies because you deal directly with the owner - there is no *middle man* taking a cut.
http://www.stilwell.co.uk/index.html
You can also buy their brochure in WHSmith, peruse it at your leisure and then ring up any suitable places to check on availability (you can do all this on line as well).