[lathspell]

Is the Ring a character? Yes, I think so. And yes, I believe the Ring is a character with the ability of making decisions and acting upon them within it's reach.
If it isn't a character, than how do you explain this?

Though he found that the thing needed looking after; it did not seem always of the same size or weight; it shrank or expanded it an odd way, and might suddenly slip off a finger where it had been tight.

This is Gandalf speaking to Frodo in 'the Shadow of the Past'.

The Ring expanded or shrank and slipped of a finger where it had been tight. How can this have been? Fingers don't suddenly get smaller. This is where the Ring's ability for decisionmaking is most perceptable. It chooses to expand or shrink. And when you make decisions, you have a purpose. And when you have a purpose, you have a will.

One reason I've been thinking about today is that of why Sauron would create something which is possessed of so much of his power and 'allow' it to have that kind of sentience?

It wasn't really a threat to Sauron. It was him, in a manner of speaking. There was, after all, only the remotest of remote possibilities that the Ring could be used against him in some way. Of course, it was, but it was still a fool's chance at that, and it did not really have a lot to do with the sentience of the Ring. In fact, the sentience of the Ring was part of its defense against that sort of thing.

In this case I agree with Kuruharan. The Ring was never made by Sauron with the intention to loose it. The Ring was never made to fall into the hands of his enemies. Sauron made it with the purpose of domination. It wouldn't have occured to him that he might loose it, just as it didn't occur to him that his enemies might seek to destroy it.
And after he lost it, the abilities of the Ring proved the best defencemechanism that you could wish for. The Ring tried to find Sauron as well as Sauron tried to find the Ring. The Ring tried to leave Frodo, using Boromir as victim. The Nazgûl were never far off. But it couldn't leave Frodo by it's ability of expanding or shrinking, because Frodo always kept it on it's chain. A ring in a ring.

Because I think that the losing and finding of the Ring is more to do with Fate, and less to do with the Ring itself choosing somebody; surely if the Ring can 'choose' then it might have chosen more suitable bearers for its own purposes?

Now you mix up to things. Choosing to leave someone is quite a different thing than choosing the next person that picks you up. It left Isildur with orcs at hand. If it was the Ring's intention to be found by them, I daren't say, because it would be a rash action to leave Isildur in the Anduin. However, it did abandon it's master's enemies.
After this comes Gollum, who takes it for his own after murdering Deagol. The Ring abandones him in a place full of orcs (and in a time when the Necromancer is searching again near the place where it was lost). It was picked up, as Gandalf says, by the unlikeliest person imaginable.
The history of the Ring from Isildur's fall on confirms the theorie of the Ring being able to abandon someone, but not being able to choose it's next master.

See? My point here is that look what conjecture does. Why is the ring a "he", and not a "she"? Well, since you have already granted sentience, why not?

Well, if I have to choose between 'he' and 'she' it would logically be a 'he'. The part that was put in it was Sauron's. And Sauron was male. Though in this case it was unintended to give the Ring a gender and I will in future continue to reflect to the Ring as 'it'.