There is a definitive answer on Customs & Excise webpage. Look at Directive No. 8
Basically states that if the boat was built prior to 1984 and was in the EU Dec 31 1992 Then OK. If Sweden or others that joined later then the dates are changed to 86 and 94, I think. Same with the countries that have just joined in the last couple of years. Look on C&I website. If you want a precis then I have it as a WORD DOCUMENT that I can email to you.