w126 Ignition switch replacement
 

Before i get any stick, i've spent the last 2 hours trawling this site and a sister site in the UK but am still unclear ....
Problem; definitely ignition switch fault as key no longer 'returns' from starting AND starter motor keeps running even when key is removed!
Battery disconnected but starter starts up again as soon as connection remade at battery (ie keys are now superfluous!)
So, what to change? Assume the electrical part of the whole ignition assembly is shorted somewhere but can I just replace this or the whole assembly?
Suggestions/experience please

1983 W126 280SE M110 Blue