Best to call them up before you do anything, I did and got an offer, though it was for the same subject I wrote about in my PS.
the system has to be fair for those who have had to settle with uni's that they didnt want to go to
They also know they're your 7th (or more choice) which can look bad!
with UCAS extra can you submit a new PS or is the same used?
However, if you contact UCAS and explain why you want to upate it, they may allow you to submit a new copy (they did with me). They're only likely to do this should you wish to make dramatic changes to your course choice (i.e. heading in another direction, applying for something in an un-related area).
I'm not sure that adding details about work experience really qualifies. There's no reason you can't talk about upcoming work experience in your original Personal Statement anyway.