STM32CubeMX 4.26.1
Despite minor odd behaviour when stepping through the code, (highlighted line in source goes backwards one line when single-stepping), the debugger it seems to now work although I am unsure as to why I was having problems.
At one point it did look like there was another instance of the workbench running in task manager which may have contributed in some way, or the usual reboot may have fixed the issue I was having.
EDIT - Duh!, the problem surfaced after updating the F1 firmware package to 1.6.1 which changed the code optimisation to Optimise Most (-O3) from None (-O0).
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