Online availability can be provided by WordPress Plugins or a simple Google Calendar.
Companies providing Booking charts with full or partial service are
InnStyle (teaming up with Booking.com)
… just make sure they have channel partner management (LateRooms, Expedia etc) so you have a wider “reach” for selling your rooms.
DIY HOLIDAY COMPANY
Freetobook offer the option to amalgamate more than one property into a Two Centre, Three Centre, Multi Centre website. They will build the framework and run the bookings for you. You could even run your own Area Holiday Group
Simply call Freetobook and ask them to host a GROUP WEBSITE. For an example of what can be done have a look at Ayrshire B&B