Hi Dafydd
You have highlighted a massive problem with postgraduate research, you have absolutely no sound way to figure out how well you are doing, whether you need to work more or less etc.
It's probably made particularly difficult by being a part timer working predominantly on your own, but having done a FT PhD I had no idea how I was doing untill 3rd year, when I had to pull my finger out big time. Now I'm a post-doc, and the immortal question still remains, I'm 8 months in, I've learned a few handy techniques and have got some decent data, I may appear on two papers but the main body of my work will probably count for nothing due to problems with the reproducibility (with the comercially available proes i'm using). My boss seems ok with me, but he's quite easy going anyway.
Best thing for the first half of your PhD is probably to try and make progress as much as possible, but keep something in reserve, as you may have to go all out in the final year to get the job done.
I hope this is of at least some use to you