@pbacot said: a "remove lonely vertex" plugin should clean it up. e.g. CleanUp³ by Thomthom. If they are not coplanar and not welded, you can have a look at Thomthoms select curve plugin.

Thank you all for your input on this. I am aware of the problems caused by not being absolutely precise in making sure lines do meet at a point and not just sort of at a point. Hours spent finding where the offending point was. This raises another query but I will start another thread as it is not related to this. Thanks again

Sorry it's taken so long to thank you, John, for your input. I have not followed our proposals - yet - but I have noted what you have said and will in future if I need to input a jpg file. Thanks again and to all who contributed.