Cook the potatoes in a pan of boiling water for 10-12 minutes, until tender. Then drain and set aside.
Melt the butter in a large pan and add the potatoes and spring onions. Season well. When ready to serve, combine with the rocket.
To make the dressing, pour the olive oil into a small pan, add the lemon juice and capers, season and heat through for 3-4 minutes, then stir in the chopped basil.
When ready to barbecue, score the skin side of the mackerel with a sharp knife, dust in seasoned flour, shake off the excess and drizzle with a little groundnut oil, season, then place skin side down on the barbecue. Grill for 2-3 minutes, turn over and grill for another 1-2 minutes, until just cooked through.
Serve drizzled with the dressing, along with the potatoes and some lemon wedges on the side.