If it's not too late, I've got a simple recipe for a SYN FREE sweet & sour sauce. I actually found it on the recipe thread (THINK it might be under 'Chinese sauces')
1 carton of passata
6 TBSP sweetner
6 TBSP malt vinegar
1/2 pint chicken stock
soy sauce to taste
1 thin red chilli, diced VERY small - seeds removed
Mix the vinegar with the sweetner.
Put the passata in a pan with the chilli (optional, but it's quite nice and not too hot).
Add the vinegar/sweetner mix, to the pan with the passata and chilli.
Stir.
Add the chicken stock.
Add salt/pepper/soy sauce to taste.
You can thicken this with cornflour, but if you let it simmer, it does go a bit thicker.
Alternatively, you can add some fine grated carrot to the sauce. If you let it simmer you don't get too much of the carrot taste.
Hope it's not too late. Enjoy!