Hi shivvy
Sorry it took so long to get back to you. I couldn't find the recipe in any of his books so I have re-watched Saturday Kitchen and fast forwarded until he came on. This is what he said he used:
1 rack of Welsh Saltmarsh Lamb, trimmed roast 20 min
Flageolet Beans....he didn't give a quantity!
2 finely chopped shallots
2 cloves garlic, crushed
Fresh Thyme (2-3 sprigs)
1 Bay Leaf
2 Peeled carrots
Peas
Olive oil
Fresh parsley chopped
Place beans in a pot with the shallots, garlic, thyme, bay leaf and carrots. Cover with water. Simmer gently and cook until the beans are soft. Strain them but keep the cooking liquid.
Put the beans back into the empty pot, slice the carrots into it and add some peas, olive oil and butter. Season with salt and pepper.
Season the lamb and roast at 180-200 for 20 mins
After 20 mins, place the rack of lamb on a warm place and cover to rest.
Pour the fat from the lamb away and place the tray on the hob. Add the cooking liquid from the beans into the pan and heat through. Strain into the bean mixture and put back on the heat and warm through. Add some chopped fresh parsley.
Carve the lamb and serve with the beans.
I am so sorry there are hardly any quantities but he kind of flies through it!