My initial invoice was for £83.45 for a LCD WiFi 3G model. (Incorrectly invoiced at $525 total)
After getting Notion Ink to send me an accurate invoice for the $475, I have phoned TNT twice to get the VAT invoice amended. Didn't get a response but yesterday I emailed: NTH.firstname.lastname@example.org attaching my correct invoice from NI and stating that the VAT needs to be recalculated. I also queried the DPF charge as it stated from GATI on behalf of NI, saying that if the charge is from GATI then I have already paid this in the $50 shipping and handling fee. I got a response today via email saying that the DPF fee will be waived and the VAT has been re calculated with the correct invoice.
From the initial £83.45 I have been credited £24.19 (£6.19 VAT credit + £15 + £3 DPF credit) I therefore have just paid the invoice at £59.26 which was around the amount I was initially expecting. So all resolved, only took 3 weeks of emailing and phoning around :P