You can use the 1 syn = 20 cals rule as theres no free food.
I've just typed "pizza express salad dressing" into google and found a website with the pizza express at home bottles. The light caesar salad one has 348 calories per 100ml, assuming 1 tbsp is 15ml, that would work out at 50 cals per tbsp or 2.5 syns.
there are other flavours as well, if you want to look up those, just do a search like I did and you should find the website with all the details