The trust had income tax to pay for 2015/16 which was paid on time in January 2017. It has no tax to pay for 2016/17 (the trust income is rents and in 2016/17 the expenditure exceeded the gross rents). HMRC and ATT’s guidance both say that a taxable relevant trust is one that paid [income tax] “in" a tax year. The regs (http://www.legislation.gov.uk/uksi/2017/692/regulation/45/made) say the same:
“a taxable relevant trust is a relevant trust in any year in which its trustees are liable to pay [income tax].”
January 2017 is, of course, in the 2016/17 year of assessment and therefore to this simpleton there is a requirement to register by 5 March. Agreements and disagreements are gratefully sought.