those look like deliberately machined out areas which are done to balance the armature at manufacture, not a problem .
Thanks your Reply Spanner Monkey.
Shortly after I posted the Photos above I discovered the reason behind the problems I was having.
The Internals are actually almost new and someone has clearly Stripped and Rebuilt the Starter prior to my Ownership.
Even the Grease Cap is as new.
However, when I looked again at how it had been previously reassembled and also spotted the tell-tale Rub Marks inside the Housing, all became clear.
There are two small Washers on the end of the Armature Shaft.
One is a Spacer Washer which lives between the Armature Bush on the inside of the Housing and the Housing itself.
The other is a Shim Washer which lives below the "E" Clip on the outside of the Housing.
Both had been installed on the outside.
When the Working Parts became warm as the Motor was running, they naturally expanded and the expansion was such that it caused the Armature Bush to rub against the inside of the Housing, restricting the movement of the Armature itself.
Hence, when the Bike was cold it turned over as normal but when it got hot it would barely turn over on the Starter at all.
Once I had rebuilt the Starter with the two Washers in their rightful places, the problem went away.
Unfortunately I didn't discover this "Revelation" until I had ordered a new Starter from Germany so now I have a Spare.