Couple of things when you open a part file or a new part.
Check the color settings - Tools/Options/Color
Check the font size - Tools/Options/Document Properties/Dimensions
If you find the solution then save the part template again to keep the settings
Thanks for the responses but it seems that it was none of the above.
On opening of other sketches in other parts there was a whole host of other anomalies including infinite dimension leaders.
This pointed to a system error rather than a setting issue and a repair has now resolved the problem.