Thanks for the advice everyone :)
First, I'm probably not technically underweight. I haven't weighed myself for ages, but I generally hover around a BMI of 18.5-19. I've been the same weight since I was 14. It's not for health reasons that I'm keen to put on a bit of weight, it's because I'm 37 now, and being thin is making me look really ropey! My face always looks tired and drawn, and plenty of people tell me how thin I am (when, probably, they would not dream of mentioning it if they thought I was too fat, but that's another issue).
Also, I don't put on weight around my middle, as such - I don't have a spare tyre or 'muffin top'. My belly just looks bloated if I eat too much.
Amelia, your asking me to write down what I eat has made me think quite carefully about my diet. Here's what I ate today, which is reasonably typical:
Breakfast:
4 slices of toast (multigrain bread) with quite a lot of butter.
Lunch:
Huge plate of pasta (probably double what a 'normal' person would eat) with a sauce made from tomatoes, olive oil, quorn sausages, onion, garlic, and philadelphia.
3 chocolate bars (@ 210 calories each)
About 50g pistachio nuts
Tea:
About 50g pistachio nuts
Big bowl of muesli-type cereal (oats, raisins, almonds, hazelnuts, sunflower seeds, honey) - probably double a normal portion - with full-fat milk.
Drinks:
Decaf tea with full-fat milk, water, squash.