Visited for afternoon tea which was booked. Told ...

Visited for afternoon tea which was booked. Told them at time of booking about my severe allergy to fish, and again when the waitress seated us. My 2 friends always insist on no fish on the table too, due to my allergy. The afternoon tea arrived, it looked delicious. We had eaten 2 sandwiches, when my friend spotted bits of salmon on the plate! We summoned the waitress who apologised, took plate away. No problem with her response. Why was there Salmon on the plate? All food stuffs are supposed to be prepared on separate chopping boards? So this means the sandwiches must be stored on same plates? Or are they recycled when left by other customers?
Upon leaving, I spoke the the employee who was taking our money, and explained about my severe allergy. Her only response was a 5% discount! I'm sorry but this is disgusting. Thank god I didn't eat any of the salmon accidentally, don't think the Celtic Manor would like an ambulance attending because I've gone into anaphalactic shock!