Error on first VAT return excluded pre registration expenses

My client has just filed their first VAT return, but has made a mess of it - the numbers they filed were taken BEFORE all of the adjustments for pre registration expenses.  I don't do their VAT returns as they didn't want to pay additional fees, but I did help them to pull some of the numbers together.  They now want to know how they file an amended VAT return.

I have a horrible feeling that they can't.  The numbers which were filed were not incorrect for the VAT quarter in question, they were just missing the pre registration piece, so I don't think they can go down the error route.  BUT I think they are only allowed to include pre registration expenses in their first VAT return, so they also can't simply do a catch up in their next VAT return.

All advice welcome...