Lighter lamb hotpot

Name: Lighter lamb hotpot
Ingredients: 3 tsp olive oil; 600g/1lb 5oz lamb leg steaks, all visible fat removed (500g/1lb 2oz after the fat is cut off), cut into bite-size pieces; 2 onions, roughly chopped; 2 garlic cloves, thickly sliced; 250g/9oz celery (about 4 stalks), thickly sliced; 400g/14oz carrots (about 4 medium carrots), peeled and thickly sliced; small handful fresh rosemary, finely chopped; few sprigs fresh thyme; 1 tbsp plain flour; 1 beef stock pot diluted to make 500ml/18fl oz stock; 2 tbsp Worcestershire sauce; 2 tbsp tomato purée; 650g/1lb 7oz floury potatoes, ideally Maris Piper, cut into thick slices; salt and freshly ground black pepper