Thanks for all the feedback guys. Sorry about the slow reply - I haven't been online much this week!
I think I've found a novel solution to him eating the grass after I've picked up - I just pick it up and dump a bucket over the site. He seems really confused by it - he just has a sniff around, and then generally just strolls off and leaves it. I'm hoping that keeping this up will de-sensitise him a bit. I'm giving him lots of praise and a treat if one's handy when he leaves the site alone.
As a rule now, he is not allowed out on his own in what we are now calling "the danger zone" (the times he usually does his business). Inconvenient and annoying, but it can't be helped I suppose.
Since doing this (since the weekend, really), he now seems to be getting a bit more interested in other dogs' mess. It's hard to tell, because I walk him off the lead, and mostly, in the dark at the moment, but I think he finding some on walks.
As for the diet thing - he does seem a bit skinny at the moment, although to be honest, I'm not sure how big he should be. He has pretty thick fur, so I can't really see his ribs, but I can feel them. I understand that this is normal, but I do wonder if maybe he's a bit hungry and this is making the problem worse. I am feeding him towards the higher end of the scale on the food packet (he has kibble - about 440g per day, plus a few tidbits/treats most days). Perhaps I'll up his food a bit.
I wanted to upload a pic for someone to comment on his weight, but I don't really have anything that shows his weight if that makes sense. This is the closest to a belly shot that I could find: