The answer above is technically correct, however, I found an additional thing I had to do to actually stop my laptop from entering sleep mode.
I had previously selected the Performance power plan, but recently discovered there were a few options not being honored. So just on a whim, I thought I would try setting my laptop back to the Balanced power plan and adjsuting its settings to suit my needs. Lo and behold the settings all of a sudden all started being honored. I also noted that the previously selected Performance power plan disappeared from my list of power plan options. So it would appear one of the recent Windows Updates has potentially removed the Performance power plan from the default list of options, and I assume therefore only offering partial backwards compatibility.
Hope this helps anybody still struggling with this annoying behavioural glitch.