Well spent a wet afternoon dismantling and re-assembling a NCDC 2013 and its mate.
Bought what I hoped was a working NCDC 2013.Took it apart and removed the nav mechanism complete and then removed the same from that in my car and swopped the 2 and re assembled.
Had high hopes and with fingers crossed refitted head unit and all worked ...except the nav part..same fault as before disc just gets drawn in and after about 30 seconds disc gets spat back out and unit askes me to insert disc.
Have done this around 10 times each with the same result so conclusion is :
1)..Either original diagnosis from Gerners was correct and something un repairable is the fault.

2)..Lightning has struck twice and headunit I bought for £30 has the same fault as mine.

Think Im now chasing a lost cause ,unless anybody has any pearls of wisdom.
On an aside the removal and refit was quite straightforward ,except the very fiddly little clips that hold the 2 ribbon cables into their sockets ..Very brittle and bits broke almost just looking at it but all replaced so did at least prove to my self I could do it