This has been very interesting indeed! From a glance at the dimensions of Poole Harbour, I would at first guess that the situation is much, much more complicated than first appears.

I Googled "double tides" and got this great link for a place in Scotland, which at least states that it's due to the what we call the bottom topography (which is what I suspected all along): http://www.rampantscotland.com/know/blknow31.htm

I think to get a good, definitive answer to your question may take some hunting in research papers. Perhaps someone's done a model of the water level and currents in Poole Harbour for a thesis or something. Indeed, from the bit I've read in my book (an introductory oceanography book with merely a single chapter on tides) it would be quite complicated! And from looking at the map, with two sets of narrows and Brownsea island in the middle - I'd say it must be quite complicated - no simple rectangular box for this model!

Edit: Oh! Oh! I've found something great, and not technical! Try http://www.weston.org.uk/tides.htm. (I am reading it right now, haven't finished it yet. Just wanted to post in case you were still on Board!)