@Chud that could well be the case, I looked at the ends and they do look at bit burred but could be degradation rather than having previously been joined
thanks very much for the advice everyone and sorry for the delay coming back, had a busy weekend. I bought a voltage tester, turned electricity to the shed off and tested the cables. then I turned it back on and tested them and there was still nothing. so I pulled out my tumble dryer and found the main (armoured) cable coming in. tested with elec off and on again and this was live, had a look at consumer unit and there was a (newish) armoured cable going into the ground so figured this was the new shed line.
back in shed, pulled out tumble dryer on other side and found cables corresponding to the white and grey ones above, tied off in the corner. checked these for current too and they are dead.
my conclusion is that the electricity was relayed and these cables were left in situ but they are disconnected from the supply.
@sparkwright I will fill the ends with mastic to prevent water ingress. the end of the trunking isn't accessible from inside the shed but I'm going to take a look under the kitchen units to see if the trunking is under there and fill with mastic if it is.
thanks everyone for the advice again. pleased I don't have to deal with this but out of interest guys, if these had been the live cables, how could this have been rectified without taking up patio to access the cables and bury them properly. There is little to no movement in the trunking - the ground is very solid and the gap between patio and shed is very small.