We used a night nanny for our twins from nightnannies.com (can pm her details to you but think she only does nights). Or one of my friends had a maternity nurse who came from up north specifically and stayed with her for 4-5 weeks, she is willing to travel anywhere afaik - let me know if you are interested in the details
We live in SW London too.
Tbh it's up to you but I thought a night nanny 3 x nights a week ie every other day worked really well for us. I would have hated having someone there full time, but then again I had 4 weeks of DH on paternity leave so we didn't really need anymore help.
I'm really looking for someone full time as I am going to have a c-section, I already have a two year old and DH may only get a few days paternity. Would be great to have the contact details for your friends maternity nurse. I am registering now with an agency, but thought it would be better to have a recommendation. Thanks for your reply!