Hey, cognitive neuroscience is my field. I guess you are a UK student? Though I'm a bit confused about the undergrad distinction thing.
Generally in psychology/cognitive neuroscience doing a masters is the norm. But you'd be a very good candidate for funding so I'd suggest to do the masters as part of a 1+3 studentship. As an RA, did you ever work on any neuroscience research? If not, the masters year would give you a chance to pick up a lot of the skills you need - programming experiments, training in MRI data acquisition, data analysis etc. Knowing this stuff can save you a lot of time during the PhD.