What is inside the Prawn Cocktail crisps flavour?
Assuming you're talking about Walker's brand, we can see the following ingredients list from Tesco.co.uk (should match what's on your bag):

Prawn Cocktail Seasoning contains: Flavouring, Sugar, Glucose, Salt, Citric Acid, Potassium Chloride, Dried Yeast, Dried Onion, Vale of Evesham Tomato Extract, Colour (Paprika Extract), Sweetener (Sucralose)

Nutritional yeast (which is what I assume the 'dried yeast' is) is often used in vegan recipes to simulate a cheesy flavor, so it's probably not that. You can imagine what sugar, glucose, salt, citric acid, tomato extract, and dried onion taste like. Potassium Chloride is commonly used as a salt replacement to lower the salt content of food without losing that salty flavor.
I suspect it's that first ingredient, 'flavoring'. You're probably talking about a product like this: http://www.alibaba.com/product-detail/prawn-Flavor-O5557-used-in-puffed_1699947706.html?s=p

prawn Flavor O5557 used in puffed foods

Professional and mature R&amp;D team, Application team.
Manufacturer of flavor

Origin: Jining city, Shandong Province, China.