Reverse charge transactions are supposed to be accounted for outside the Flat Rate scheme, the same way as you would if you were not on flat rate, so you simultaneously “pay” the VAT as if you were the supplier and “reclaim” the vat as if you were a normal (non flat rate) VAT registered customer. So the net effect of the purchase on your VAT return should be nil - adding the same amount to both boxes 1 and 4, so no change to box 5.