Roasted peppers from a jar make a great substitute. Two small jars or one large one should give you a similar amount.
Heat grill to High. Put the peppers on a baking tray and grill until blackened, turning often to evenly colour. Put into a bowl covered with cling film to cool.
Cook the pasta following pack instructions, then drain and cool under cold running water. Set aside. Make the dressing by whisking together the vinegar, oil and some salt and pepper. Stir in the capers, then fold half through the pasta and tip onto a platter.
Once the peppers are cool enough to handle, peel off the blackened skin. Remove the seeds, slice the flesh into strips and add to the pasta. Flake on the tuna and scatter the olives and rocket on top. Finish with the remaining dressing.