Hear me out, along-side your PhD you are allowed to work 20 hours part-time. My PhD is in engineering, my background is maths and physics and tbh, i mean there's always volume to a PhD, but I find it relatively light work due to my background. My reasoning for wanting to do is I am currently doing a PhD in applied math/physics within the department of engineering, but really want to do mathematical physics tbh.

The only reason I would not want to do this is if after a PhD, I have a greater chance of getting into mathematical physics than I did for getting onto such a PhD programme (I did get quite a few interviews, but never quite got one), which imo, though others will know better than me no doubt, will certainly not be the case- my guess is that having a PhD in a different area is definitely not going to make you a stronger candidate.

If I did a Mres alongside, I would (hopefully) get some publications and then my guess is prior degrees would not matter as much or?

In terms of PhD advisors frowning upon this- may sound silly, but do they need to know? If you get a part-time job along-side your PhD for example, you are not required to disclose this information to them. Also wondering when applying for the Mres programmes, would I need to say that I am currently doing a PhD, would this possibly affect the decision (baring in mind it is a part-time one I am applying for only)?

Also anyone experience of how Mres works in terms of logistics of meeting your supervisor. My guess is that most online meetings and face-to -face meetings occasionally needed? I am thinking if I did the Mres at a nearby uni 1-2 hour commute (or even my own - it would be in a different department). And my PhD does not require me to be on - sight at all, other than fortnightly meetings, which, ofc, have yet to go face-to-face due to Covid - I am 2 months away from a year through my PhD.

Many thanks !!

Look into the requirements at your chosen uni- I know from a friend doing a Mres, that she has to be completely finished and off the master register in our uni before she could join her now uni for a PhD programme.
Personally, I think they might frown upon it, it could be seen as you not be really interested in your current PhD. In terms of them not knowing, if the Mres is funded who are you using as a reference? I started out on the Mres and transfer to PhD in year 2- they work the same and require the same commitment.

I commented on a related thread of yours, I think. So, a few comments:

1) You might be able to. A friend of mine did a Masters at the same time as her PhD. She received special permission from her University to do it, but even then she found it incredibly difficult and ended up dropping out of the Masters (which was not cheap).

2) I don't know where you are in your PhD but the workload in year 1-2 is completely unrelated to the workload in year 3-4. You'd be setting yourself up for a situation where the hardest parts of your PhD will coincide with the hardest parts of your Masters degree.

3) Working 20 hours a week grading papers or tutoring kids is not the same as spending 20 hours a week trying to write papers in general relativity alongside your day job.

4) What you want now is not necessarily what you will want in a year. In the second year of my PhD I was bored out of my skull and absolutely desperate to change fields. Part of this was because I thought my current work wasn't challenging enough. In reality we only have a superficial understanding of our work in the first few years of a PhD.

5) The latter is especially true when we only do what our supervisors tell us to do. If you immerse yourself in the literature and start pursuing your own projects you may well soon realise that the field you're in is perhaps far more challenging -- and interesting -- than you realise.

But in the end if it's really important to you it's worth looking into and taking seriously.