I'd add a couple of other dips to the houmous - tapenade maybe? Are you putting salad dressings, mayo, ketchup, mustard etc out?
Rolls/baps for burgers? A premade pan of onions kept warm on the BBQ?
I'd also do a large marinaded cucumber salad or tomato salad just in case there are any picky people like me who don't like creamy stuff. Cucumber salad can be made in advance, best made in advance in fact, just take 3 or 4 large cucumber, slice lengthways, deceded, halve length ways again and chop into cubes. Gently heat vinegar and sugar in a pan, cool, pour over cucumber, chill and mix occasionally.
Don't do carrot salad, the grating takes forever and you may end up with blood everywhere.
Ooh and couscous salad always goes down well at bbqs and its dead easy. Roast mediterranean veg in olive oil and dash balsamic. Make up couscous with boiling water. Add roasted veg and red pesto to taste.
The menu you've got looks really good. Love the frittata idea. Have you got a separate BBQ for the vegetarian food or are they all ok with it being cooked on the same grill? I have some veggie friends who are ok with it and some who definitely aren't.
Big potatoes cut into, err, wedges. I don't parboil at all. Cover with lots of olive oil, then sprinkle with crushed coriander seeds, a generous amount of turmeric (they look all orangey and yum), a bit of paprika, pepper, and most importantly salt - but it has to be big nice crystals.
Ground cumin too if you want.
These are SO easy to prep, then you pop them in the oven just as guests arrive, then an hour or so later they are ready. Great to be able to bring out something hot and cooked that doesn't depend on the bbq and that you are in control of iyswim. Do lots.
I'd also probably have gin and tonic available for any non-beer or wine people, not too pricey!
Looks great! Totally agree on the couscous salad but I'd use giant couscous or orzo, solely because I always find it hard to eat regular couscous with just a fork - it goes all over the place.
The only dip from sainsbury's I like is the onion & garlic dip, but really I prefer jarred salsa with tortilla chips or hummus.
Frittata sounds great - the last bbq we had I made a spanish tortilla which went down well as you can cut it into wedges and eat it with your hands, so I can say with authority that it will work well at a bbq (esp for veggies).
Just to add to the above, we buy sainsbury's basics and just add a bit more lemon juice and olive oil. Decant into your own bowl and add a sprinkling of chickpeas and paprika too the top to give it a more homemade look if you like!
I would go for quorn sausages over the burgers personally. Grilled halloumi is fab and you could add that to the kebabs if you want. I def think you need to give veggies some protein, so not just mushrooms with a tiny bit of cheese.
I"ve given up burgers and sausages and anything individual on the bbq. I get butterflied lamb legs which marinade in mint and rosemary before going on the bbq or a big rump of beef which doesn't get pre marinaded but gets sprinkled with lemon juice and salt after its cooked.
I find a lot of a little is preferable to a little of a lot.
I realise that doesn't really help you very much at this stage but an idea for the future.