There are three key sources of advice and help if you have questions about Intrastat and Supplementary Declarations (SDs).
Your first port of call would usually be HMRC VAT Helpline on Tel 0300 200 3700 which can help with most queries.
You can get more detailed information on Intrastat and SDs from Notice 60, which covers the processes in greater depth. Read Notice 60 Intrastat general guide.
HMRC Notice 725 the Single Market
If you want practical help on how to use the Tariff to classify your goods, you can contact the HMRC Tariff Classification Service by emailing firstname.lastname@example.org. Read more about classification of goods.
You can contact the Excise and Customs Helpline on Tel 0300 200 3700.