Garden Rear access

I bought the final tranch of a shared ownership property 4 years ago. Whilst completing I was advised to cross out and initial the access rights to the rear of the properties garden. I have since had an extension built on the rear of the property and would like to have the access again as the extension built which blocks my side shared access?!! How do I go about this?

Philip - speak to whoever owns the land over which you want access and/or whoever you dealt with re the shared ownership purchase and 'crossing out' of the access right.
It reads very much as if you need legal advice to establish the right access. And after then, if granted, registering that right (easement) as appropriate. Your legal advice can assist there also