Blob, I could have written your post. I've lost, as of today, three stone one pound on Cambridge SS, but have been feeling demotivated and grim about it all for a few weeks, although overall, I'm happy with the programme. It's been instructive to take food - and emotional eating - out of the equation entirely, though I'm not wild about living on sachets -though it's temporary. So no little well-meaning lectures from me on how terrible Cambridge is, and how you should just be eating less and moving more.
I still have a stone and nine pounds to go to my target, and I think you're right that, having lost a good chunk of weight, the urgency of the motivation lessens slightly. It's probably like the middle bit of a marathon.
I'm feeling good right now this second purely because I thought I might actually have gained a pound this week, but to my delight, I'd lost three pounds, and finally got over the three stone mark, after very small losses for weeks. I'm now feeling good enough to hold myself to sticking rigidly to the diet for a week, to see if I can do better.
I wish I had better suggestions for re-kickstarting. In my case, the better I do, the better I do, but a bad day makes me want to shrug and give up. One thing I do find helpful is nice non-food smells, like herbal teas, bath salts, expensive hand cream.
Good luck! How much further do you have to go?