Part-time PhD workload compared to part-time undergrad workload Does a part-time PhD workload differ significantly from part-time undergrad studies? 

I'm intending to go on to postgraduate studies next year. I'd prefer to go straight to a PhD if possible, but will consider Masters if it looks like the more tenable option. I've worked full time for the past 9 years while completing first a certificate, and then a BSc which is currently being wrapped up, both via the Open University in the UK. 

During this time I haven't found my studies to be overly burdensome, even when taking two or three modules concurrently. I don't mind spending evenings and weekends reading, researching, and performing practical coursework. (Don't get me wrong, it's hard work, but worth it!)

This question: Is it possible to work full time and complete a PhD? discusses the relative difficulty of working full time and pursuing a PhD. My question is slightly different, though, as I'm already used to having less personal time than others might expect. 

Should I expect my personal time to diminish significantly when I continue on to postgraduate study, compared to life as a part-time undergrad student? 

I'd particularly value answers from previous full-time-working part-time OU students who continued on with a part-time PhD. 

My intended discipline is Computer Science, and I'd like to attend a University in the UK, particularly Sheffield, but as I haven't completed my BSc and begun applying for postgraduate positions yet, I can't be sure of the location.
A bit late with this answer but hopefully will be helpful. You ask;


  Should I expect my personal time to diminish significantly when I
  continue on to postgraduate study, compared to life as a part-time
  undergrad student?


Judging by my personal experience the short answer is yes.

Here's a bit more detail though. I am studying in Ireland, so not to dissimilar to the UK system. I work full-time and also did an undergraduate degree, part-time, before progressing to post-graduate study. My degree and post-grad are not related to my work but something I have a lot of passion for. I found with my undergraduate degree I had a lot more free time. Lectures were 3 nights a week for year 1-2 and 2 nights a week in years 3-4. Speaking for myself on top of lecture nights I found I could usually get away with doing a bit of extra work at the weekend. 

Two years ago I started a History Masters(which I have just transferred to a PhD) and found that almost all my free time is now used with doing my PhD work. Saying that it's important to have other activities outside of your full-time work and study. I follow my local football team so usual have a evening off each week. 

As you mention you have not decided on location so I'd like to add something about my experience. My workplace is about a 5 minute walk from my University. I think this makes all the difference to me. I can leave work at 5pm, have a coffee and be working on my PhD by 5.30. I also have my own dedicated desk space there which is a benefit with regard to having a space to leave books etc. Together with working in a family business this also gives me the flexibility to be able to arrange meetings with my Supervisor at working hours times and to pop over to the university during the day if I need to. So if possible I think it would be good to consider location of your institution in your decision making.

Just to sum up my weekly time spent on my PhD (and this can always vary depending on other commitments.) About 4 nights a week 6pm to about 10.30pm, Saturday 11am to about 10pm and Sunday about 1pm to 7.30pm. 

We have the OU in Ireland too. The university I go to is as StrongBad describes in their answer a bricks and mortar university. In my own situation I have found every department that I have dealt with to be very understanding to the part-time nature of some of its students and to be honest the only situation I can think of I have been disadvantaged by not been a full time student is the opening hours of some of the offices. That said most functions of the college can be done online now or I can email the respective office.