I completed DH's on-line form last September and when it came up with a tax owing calculation at the end I assumed (very foolishly!) that it had been sent.
We had a letter from the IR earlier in the year saying they hadn't had it - I assumed (even more foolishly!) that this was a communications error, but there were 2 details on the return that needed correction anyway, so DH rang them - he was supposed to confirm that they had had the on-line form and I don't think he did (didn't write it down...) but he did give the person he spoke to the 2 corrected figures (one of them was DS2's birthdate, how shaming) and she didn't say "but we haven't had the form" - did say "put it in writing" but he forgot.
Now we have had a further letter saying that we have to pay a £100 penalty for late filing. DH rang the IR again today and was told they hadn't received it but that "a lot of people have been confused by the on-line service". I went back to the internet site and there was the form still waiting to be submitted. I have done it now...
I am a bit cross because it should have been clear to the person he first spoke to that something wasn't right - I don't think people would normally ring the tax office to give them one figure and a birthdate if they hadn't actually submitted the form? I could have gone back to check then if she'd told him it wasn't there.