Wow bubbles sounds a great idea.
I would also get a blanket for baby, wash in nice fabric softener and then sleep in the bed with it for a week. Ask the social worker to pass it on to the family for the baby to get used to your smell before you arrive. Whether it really works or not, no idea, but we did it for our son, who was three when we adopted him, (and also for our kitten!).
Re "I have two dilemmas rolling round my head.
Firstly what did you do when you met your child for the first time? Did you just sit down close by, sit next to them etc. Lo isn't yet mobile so won't approach us and I worried about standing there like an idiot whilst the social workers watch on." I think you've had great advice already from Kristina and co. But just remember you are not an idiot, and no one will think you are. Everyone will likely be a bit nervous and hoping all will go well.
Just be friendly to foster carers and ask some polite questions (not too many) always about baby and volunteer whatever information they ask that you are happy to give.
Re "Secondly, introducing lo to family....It is my sons birthday around 2 weeks after lo arrived so decided that weekend is a good one to introduce her to her grandparents. My parents will happily come to us but my in-laws don't really like coming to ours and will expect us to go to them. I personally feel that her first meeting with strangers should be somewhere familiar and so want to put my foot down and say all visits are in my home. Would you agree or am I being precious?"
I totally agree with Kristina, really this is so important... "I think you should make sons party only about him and do family introductions another time . Trying to mix the two is a potential for disaster . Baby will get passed around ( very bad ) and DS will have his birthday spoilt ."
You will not only be managing relationships with your parents and in laws but also with your own son, we adopted with a birth daughter aged 9 and she was very jealous at first, so be very low key about new baby, she will not miss out by things being low key for her, in fact better for her, and your son can be the king of the castle on his own birthday.
And intros generally, your child, your rules. I think two weeks in is way too early, especially if they will expect to pick up the baby. Better to wait and for you to hold baby and they can do the 'goodgey goo' talk and take a few gentle, no flash, photos. Please prepare them now it will not be like when they met your birth son. Both my mum and in laws met dd on her day of birth at the hospital. They met ds several weeks after he arrived.