Help - VAT not getting added to products following upgrade to 4.1

I was running 3.8 and decided to update to 4.1 a couple of days ago...
Now the VAT is not getting applied to products despite the Tax being the same.

In panic I tried to close the shop but that did not work either so I've had to turn it off at iis level.

Can anyone offer any help?

Each product has the correct Tax allocation and the countries are set to apply tax.
I'm at a loss and losing customers at an alarming rate.

Final question, can I roll back to 3.8 from my backup code, but retain any database data that has been added since the update (I don't want to lose the customer and order data)

Any help gratefully received

Simon

I have VAT back working now.. It turns out that the app I installed for Nop to SAGE One removed the old VAT rules and applied it's own blocking VAT.  So I've removed that and re-installed the default tax provider.

But the store close option is broken still