I agree, it's worth applying and seeing what happens - in all honesty, I think the fact that you can't start for a long while might put them off, as unfair as that seems - but you never know, if they don't get anyone else suitable they might be willing to wait.
It shouldn't affect your maternity pay I don't think - certainly if you leave at the end of your maternity leave and get another job in the NHS it counts as returning to work, so changing jobs part way through should be the same. If anything, HR might have to do some paperwork to end your maternity pay from your old job and re-do it for your new job, but from your point of view it should be exactly the same. But thinking about it, if they do hold the job for you, you wouldn't have to resign from your current job immediately anyway would you, you could leave it until 4 weeks before you want to start the new one (or however long your notice period is).
Sohappy, you're right about having to go to any NHS trust for 3 months after maternity leave to not have to pay any money back, as long as you're in your new job by 12 months from the date your maternity leave started.