Although you grew them from seed, once they become plants and grow on, they start to form tubers - these may be small initially, but will get larger year on year, if you keep them going that long. That's likely what your 'potato' objects are - assuming they're obviously part of the dahlia roots.

yes bamboo they were part of the roots, thats great to know, how do i keep them over winter, i still have 4 more troughs to clear, so do i leave them where they are or lift them and store,

i find it fascinating to discover new things

You need to overwinter them somewhere frost free - you can either leave them in their pots and put them in a greenhouse or somewhere that's relatively dry and doesn't get too cold in winter, or decant them, leave them on their sides to dry out on newspaper or something. Then knock off the topgrowth and put them in a box or container full of dry material, such as sand, or spent compost. Then you need to start them back into growth in early March by potting up in new soil and watering - keeping them somewhere with good daylight and a little warmth, but not outdoors, ready to put back outside in late May.

thats magnificent,
thats all the information i need .
thank you so much for all your advice it really is appreciated

steve