In the container of a food processor, combine the chillies, horseradish, garlic and mayonnaise. Set aside 3/4 of the mixture in the refrigerator. Brush or spread the remaining 1/4 onto the salmon fillets. Cover and refrigerate for at least 30 minutes to marinate.
Preheat a barbecue for medium-high heat. When hot, lightly oil the grate.
Barbecue the salmon fillets until they easily flake with a fork, about 7 minutes per side. Baste occasionally with the remaining marinade.
If you can't find fresh jalapeno chillies, feel free to use chillies from a jar. Otherwise, your favourite green or red chilli can be used instead - just keep in mind how hot they are! Also, The English Provender Co. sell grated horseradish.