Part Time PhD workload compared to part time undergrad.

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 an 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.