I'd be going to a postnatal physiotherapist, you won't need a referral. It sounds to me like everyone expected that it should fix itself and from what you say it hasn't.

I had this after DD but mine did fix itself. The hospital I went to had a physio come around to give me a free assessment (they did this for all the new mums) and I was told that if the gap hadn't narrowed in 6 weeks to make an appointment with the physio.