At the present time your are unable to directly amend transactions that are within a previously filed period if you are using FRS.
When you refer to amend, what are you attempting to do to the transactions?
I was attempting to change an amount and delete a line.
Has this always been the case - I thought I had been able to make changes previously?
This recently changed once MTD was implemented. You will need to make any changes as an adjustment on your open return, if this change will mean you are due to pay more or less VAT.