Unless you have been seriously taking the mick, I have alway found it depends on your tone with the person on the phone
and the person on the other end of the phone.
The VAT man is happier you are now registered, which will give them more money.
The line "look folks, I'm doing my best here'. I could stay off your radar
and in the black market like loads of people" always worked for me in the past.
They tend to take out the hatchet for late fees and penalties when you have been really acting the twit.
Always be nice to them and they are generally nice back, in my experience.