Neil, you are just at the beginning and it is totally ok to ask all these questions. But, there is no general answer to it. Shades of reality may cover all you imagine in all variants and it will depend upon whom you step into.
Roughly as a guide to your questions from my experience:
1) PA28 at 75 GBP feels like dry, so plus fuel.
2) I doubt you fit the unexpected into 80 GBP per month.
3) The way I encountered this would cover hangar etc, but not annual.
4) This really depends on the contract, it may or may not.
5) As a low timer you definitely will have an insurance issue.
One drawback, which keeps my away from shares, is the sync of expectations of different people. If you hire, you look at the plane and like it or not. If you have a share, you have to balance the one owner keen on the latests avionics with the other being a rock solid bushpilot denying all technical progress - at worst. As a low-timer you will have a lot of changes going on in your mind for quite some flying hours to come. So, my advice would be to take some time with only hired planes, hang around the bushes of your most favorite airfield, get to know people, get a feeling for our little fellow pilot-owner bastards, decide with whom you may want more contact and learn learn learn learn learn, not only piloting, but to estimate our emotional borderline-like community as well.
And a little additional advice, talk to people and drop you may want to get your own plane. By this I had this surprise to come along somebody wanting to get off his plane to a guy he likes and that was me. All of a sudden I had my own, very affordable, plane (but don't ask what the misses said ... did not tell her in advance).