How long ago was this 'canned ratatouille' purchased? Waitrose appear to have mended their ways and no longer retail such a canned product.
Or was it a bag of ingredients for a Waitrose ratatouille recipe? They have menus for ratatouille, and things called sausage ratatouille and (compounding offence) winter vegetable ratatouille.
You'd be better off with a brace of snipe (or woodcock at a pinch). Here is how to cook them
Link.