I have now changed to a 6:1:1 Sand/Cement/Lime with the sand being
Wickes (red bag) "for use in bricklaying and internal rendering". Is
this the same as plastering sand ? Would plastering sand be better in
this case ?

dont confuse 1:1:6 with lime mortar though, very different.

Can you explain the difference ? This is not 1 part sand, 1 cement and
6 hydrated lime is it ? Lime mortar uses a different type of lime?

Is it possible to create a mix the same as what was originally used ?