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.
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.