Final credit check on completion?

[Lu86]

I wonder if anyone can help...
My partner and myself have recently secured a mortgage with Santander in that we have given our deposit and been fully approved and credit checked etc and have now exchanged contracts.
I have since missed/was late making my monthly payments on a Next account (2 months in a row) and I am massively panicking as I have heard mixed views on whether or not a final credit check will be carried out on completion (we are waiting for the house to finish being built). Also if they do carry one out is this something that would be a major problem in their eyes? Help or advice appreciated!