I'm veggie, I don't eat fish, and much prefer a salad to a sandwich when I'm out and about. I've had some salads from M&S a few times, and they've been delicious. The ones I've had have been the Fuller Longer one, with beans and quinoa and feta; and a similar one with goat's cheese and quinoa. Both have lovely dressings in a pot. I've also had something similar from Waitrose.
How can I recreate these at home? Is there any recipe book for substantial veggie salads (I mean not leaves!)
Try Middle Eastern cookbooks - they always have a brilliant collection of veg recipes and lots of substantial salads. Ottolenghi's Jerusalem is one but check out others at your local library. Also Riverford website has great recipes that always work.