Preheat your slow cooker if necessary
Heat a large frying pan on MEDIUM-HIGH, add half the oil and brown the lamb shanks all over, then transfer them to the slow cooker. This will take about 10 minutes
Turn the heat to MEDIUM-LOW. Add the remaining oil to the frying pan and fry the onion gently for 2-3 minutes
Add the chopped carrot, celery, and garlic and cook for a further 5 minutes until the vegetables will start to soften
Stir in the tomato purée and flour and cook for 1 minute
Slowly add the wine and half the stock and bring to the boil, stirring until you have a smooth sauce (apart from the vegetables!)
Add the tinned tomatoes and bring back to the boil
Pour the sauce over the lamb in the slow cooker
Pour the rest of the stock into the frying pan, bring it to the boil and stir the pan to pick up any sauce, then pour into the slow cooker.
Add the bay leaves, rosemary and thyme to the slow cooker and put the lid on
Cook on LOW for 8-10 hours or on HIGH for 6 hours. The lamb should be tender and starting to fall off the bone.
Season the sauce with salt and pepper to taste
Serve with mashed potato and a green vegetable, e.g. broccoli.