Link imported data into child table to parent table (when table links have already been done)

Hmmm

Imported over 10,000 records into a child table (Pcosts - The many side of relationship)
This table was already linked to parent table (PMain - The one side of relaionship) before import.

Both tables have a common field (CustRef)

Pcosts Child table is set for referencial integrity so is a composite! (but doesn't have to be - Yet!)

Now I have nice amount of data in child table but

It has not auto linked on CustRef (the table is correctly joined in the right direction)

Albeit Ninox politely offers me the chance to manually link the Child records to each Parent form (one by one)

I think I will decline this offer.

Any suggestions? How I automate this step?