Put a large pan of salted water on to boil. Scrub the potatoes and chop them into bite-size chunks. Peel the onion and carrot. Finely chop them both. Trim and finely chop the celery.
Pour 1 tbsp olive oil into a large pan and warm on a medium-high heat. Add 4 sausages (save the other 2 in the pack for breakfast) and brown for about 2 mins on each side, then transfer to a plate.
Turn the heat down under the pan slightly and add the onion, carrot and celery with 2 tbsp water. Season well with salt and pepper. Sweat for 5 mins to soften, stirring occasionally.
While the veg cook, peel and thinly slice the garlic. Add to the pan with the tin of cherry tomatoes. Drain the cannellini beans, rinse and add them too. Stir in 1 tsp caster sugar. Pour in 150ml cold water and stir together. Cover. Bring to the boil.
Return the sausages to the pan. Partly cover with a lid and simmer for 20 mins till the sausages are cooked through and the sauce has thickened slightly. Taste and add more salt and pepper if you think it needs it.
The water in the pan should be boiling by now. Add the chopped spuds, cover and simmer for 15-20 mins till the potatoes are tender when pressed with a fork.
Drain the potatoes, then tip them back into the pan. Add the butter and a pinch of salt and pepper. Lightly crush with a fork or masher to just squash them a little. Scoop the crushed potatoes onto a couple of warm plates.
Tear up the basil leaves and stir half through the sausages and beans. Ladle into bowls and scatter the remaining basil leaves over the top to serve.