UPDATE:
After some thought and a lot of digging, we've reluctantly decided that Plakias is probably not the right resort for us.
We really wanted a resort where we could get to other places, especially a town something like Kos or Rhodes Town.

Also, landing at 17:20 in Heraklion would mean we probably wouldn't get to resort (after a coach transfer) until 9:30 pm.

So we've booked to fly to Chania and stay in Agia Marina which means we can get into Chania town which, from what I've read, seems nice.

Flight times to/from Chania are better for us too.

I've also found a thread that says you can do a day excursion to Knossos.

No buses that I know of. Sorry

Thanks, Jenny.

After doing some digging, we decided that although the resort sounded perfect for us we would not have been able to get out-and-about enough for us so changed our minds and booked for Agia Marina (see my post above).

According to the local bus timetable, the buses into Chania Town (which from accounts sounds very interesting) are very frequent and we can get to other resorts if we get fed up with plain beach days.

Chania is lovely with a superb harbour. Have a look at the webcam HERE

There is a good bus service all along the E75 North coast motorway. Make sure you call in to the next town of Rethymnon as well on the way. Another beautiful harbour. A few photos HERE.