There is not a straightforward answer, as it will depend on several factors.
Masters aren't a substitute for experience, supervision, exposure to working in clinical psychology related settings and understanding the role/ developing the capabilities required of a clinical psychologist. That material gives you stuff to reflect on in your application form and (hopefully) your DClinPsy interviews. That side needs to be priortised, and a good supervisor will give you the insight to know what your strengths and weaknesses are, especially if they have helped a few people through the pipeline.
No one needs one. However, Masters can help IF you use them properly.
I have seen students sign up, sit passively through lectures, take the easier/ 'more interesting' modules and do the bare minimum to get through it. That group may as well not have bothered, doesn't tend to fare well and are often left feeling ripped off because their MSc was a waste.
On the other hand I have seen MSc students really go beyond the syllabus and used the time to make links to researchers and clinicians, think about publication of their work rather than 'getting marks', and using that time to understand the system far better and understanding where they can shine at interview compared to their peers. That tends to work far better and they take, or even better still develop their own opportunities, which puts them in a great position when it comes to DClin applications.
This will depend on you more than anything else. How did you use your time as an undergrad? Did you publish anything, did you make links and foster mentorship relationships with staff or win awards? What did you do in your role as a MHSW? I know some of them will be attending meetings, writing up their dissertation or helping with audits, and actively making themselves useful to the service psychologists, whereas others just put their time in on a shift and the shotgun applications at the NHSJobs website (which tends to work less well).