When I insert the key fully it just turns through 360 degrees and does nothing.
If I withdraw the key about 3mm it then functions correctly and operates the starter.
It's a 2006 S type that's only done 58k genuine miles.
Has anyone else had a similar problem and what was the actual cause.

 

Never heard of it over the last 7 years.

Take the whole shroud etc off and figure out what's wrong - maybe with a spare lock ready and rebuild to your key shape.

 

Looks like I may have to replace the lock barrel.