Hi Gemma, there are a couple of websites you can find groups on. The first is Happity and the other is Mumbler. I have a 9 week old too and there are some lovely groups around, but the closest ones I know to you are parkstone which I know isn’t really walkable x https://www.happity.co.uk https://bcp.mumbler.co.uk
I used to take my LO to Ryme Time at the local library’s around Poole and Bmth, they hold one in Westbourne on Thursdays I believe.