She does help with the shopping - it's a lot of stuff for us to carry between us though and involves us both pulling a trolley each and carrying a couple of bags too. At the moment we're going to the shop as little as we can so we have to carry more when we do. DH also goes out once a week to bring in some of the heavier stuff.
Lots of food for thought (no pun intended) here. I want to clarify that I'm not keeping her reliant so she won't leave home - this is just a habit I've fallen into with absolutely no manipulative reasons why. However his evening I shall be leaving a packet of pasta and a jar of sauce on the side and she can help herself. I'll look for some simple recipes for her and she can shop for the ingredients. I have done all this with her in the past, by the way, when she was mid teens. It's not like I've never attempted to teach her to cook.
She's had a shit time lately (job loss, having to put her plans to travel on hold, feeling shit about herself generally) and I think I've indulged her because of that. She's never had friends - she was severely bullied at school and at university she wasn't interested in drinking and clubbing or socialising generally. She does have interests (she plays a few instruments, she loves crafty stuff, etc) but they're all things she does alone.
I don't think there's much chance of me getting DH to learn to cook. To be fair he does work long hours and I don't at all so I don't mind really. His mother never taught him to cook anything and he's never lived alone so he hasn't felt the need to teach himself.
To those asking whether I would do all this for a lodger - I had a lodger about 20 years ago and I did. I was told that meals and washing were 'board' and 'lodging' was rent, and you charged less if meals etc weren't included. I also used to know someone who refused to charge adult DS anything at all because she regarded asking DC for board was making money out of them! She used to clean his room, so his washing, cook his meals, take him to and pick him up from work etc too. At least I'm not that bad 
Incidentally how much do people charge adult DC for board? DD currently pays about £200pm which I worry is a bit steep but we can't manage on much less. When she was working she paid more though.
Thanks everyone for your replies, they really have made me think.