To prepare the poussin, place a poussin breast side down, on a board for raw meat use. Using poultry shears or strong kitchen scissors, cut along each side of the backbone and remove it. Either snip the wishbone with the shears, or push down on the wishbone to snap it. Snip of any excess bits of skin, the wing tips and any other ragged bits.
To keep them in shape insert wooden or short metal skewers through the bird, diagonally in a criss-cross shape. Place in a large glass or china dish.
For the marinade, pour the Young’s beer into a large jug, once it has settled, add the rest of the ingredients and whisk together. Pour half of the marinade over the poussin reserving the rest for the sauce. Cover and marinate in the refrigerator for at least 4 hours or overnight.
Bring the poussin out of the fridge 1 hour before cooking. Pour the marinade into a pan, boil rapidly until reduced to about 100ml; this will be used to baste the poussin while cooking. To finish the sauce, strain the reserved marinade into a clean pan, add the tomato ketchup and sugar, bring to the boil. Blend the cornflour with a little water, stir into the sauce and simmer to thicken.
Prepare the barbecue, lightly oil the rack and place the poussin bone side down first. Grill for about 20 minutes, turning every 3-4 minutes and brushing with the baste. Time to cook through will depend on size of the birds and intensity of the heat from the barbecue, should be approximately 15-20 minutes. Test by inserting the point of a small knife. Transfer to a serving platter. Reheat the sauce before serving.