Hartbeeps is lovely. I also did a baby sensory at the football club which was good. Witham URC church have a bumps and babies group upstairs on a weds for up to 6 month olds. It's a really friendly group, they just put some toys out and you get tea and cake and a chance to chat with other mums.
The hartbeep in Witham is lovely I go with my 4 month old and started when she was about 10 weeks