Thanks for all the replies:
Just to clarify though, the new cable was not too short as the new box was placed right beside the old box, so the, loosely strung, replacement cable appeared able to reach the old box quite easily.
And BB_TX, your description was exactly what I was planning on doing. Take photos or label current box layout and just repeat with new cable. Logically seemed pretty straightforward.
Just figured, because no-one seemed to want to tackle the junction box connection in any of the vids/tutes, that I was missing something that made it difficult.
Marv