If you've spent a day in close contact with someone who gets swine flu two days later...

[alittleteapot]

...and you don't get it, does that mean you won't? Will some of us just develop immunity? Or, for e.g. if you live in the same house as a sufferer but don't get it, why's that then?

Usually people are infectious around the time they become symptomatic, so you might have got away with not even being exposed. So many variables though; it's difficult to say with any certainty.

There are plenty of anecdotal reports of families where not everyone gets it though, despite prolonged and close contact. HPA are doing lots of follow-up so that these kinds of questions can be answered.

I read up to 7 day incubation

and its a virus - this is not the way viruses work - it can sometimes take repeated contact to contract a virus and some people will never do so - have you never heard of children simply not getting chicken pox for ages and then suddenly they do

Yes, which is why the OP might not have even been exposed on this occasion.

I spoke with a doctor about when you are spreading it and he said thatr until you have the syptoms yourself then you are not spreading it at all

so you are not definitely going to get it