Found it. It seems that by default the IWDG is on so is causing resets. Solved by including the IWDG initialisation and servicing resets to it in the main thread. Something must have changed in the startup(F103C6tx) to have done this as this project ...

Could only be a knock off chip and / or programmer if Farnell are supplying knock offs which I very much doubt. This worked perfectly prior to updating from V1.6.1 to 1.7.0. RCC_CSR shows reset being caused by Low Power management and software resets...