Sounds great that you have a definite plan for your future. I have found that in this current market and with funding cuts to uni's by government and research councils, it is extremely competitive out there. A 2.1 (which is the minimum they look for) may not be enough, if other candidates have 1st's and/or work experience.
Adding a masters degree to your CV, will add value and if you do well, it may put you in a more favourable position than candidates with mere BSc's. A note of caution though, it may also not! It all depends on the reputation of the uni you plan on studying your MSc, the quality of their research, PI (supervisor) and department in terms of neuroscience and where you apply for PhD's. The top uni's will be more difficult to get into.
I think in terms of work experience...which is valuable, especially if you want to be competitive, is a must. When I applied for PhD's last year, I only had two interviews out of many many applications...I spent months gaining work experience in research labs since and my CV is now more competitive for the market. I'm planning to complete an MRes this year and then go onto a PhD next year. I believe that my 1st + work experience + the MRes, will give me more of a chance of getting a funded PhD.
This is my opinion and based on my own experiences, so that's all I can say. Others may disagree, but let's put it this way, if you can get some relevant work experience under your belt before applying for PhD's, it won't hurt at all.