Mexican is nice, cheap and easy. Get either tacos, nachips or corn/flour tortillas and make the following fillings ...
Chilli minced beef
Refritos (pinto beans boiled then mashed and refried with tomoato, chilli, garlic and onion).
grated cheese (Manchego is good)
Guacamole (mushed avacado with chlli, tomatoes and a squeeze of lime
Salsa (in a jar)
Hi Twiglett, do you mean you are hosting the evening or are you a guest being asked to bring something? I go to loads of these things as am part of an ex-pat community in Holland and usually, everyone brings something from their own country. So for Britain I would maybe bring a traditional Brit pud like apple pie or strawberries and cream, or a savoury pie - fish, chicken? Think cooking for the Queen thingy on the telly!
Actually, just thought, I did a chicken and ham pie recently which went down well - prepared well in advance, cooked as I was getting ready to go out (just has to be bunged in oven) then was still warm as we arrived at the 'do'. Am sure others will have much cleverer ideas though!
Although its hassle, I do think its nice to have some warm things as opposed to lots of cold buffet foods. How about some small baked potatoes wrapped in foil which stay warm for ages and separate bowls of cheese, soured cream, onions?
along with the felafel and houmous, how about tzatziki- cucumber, yoghurt and mint. You could also do lamb balls (i.e. meatballs made with lamb mince and pine nuts)- easily stuff well into pitta pockets