There is no such letter. But just send them the information.
Council Tax is subject to its own limitation period under reg 34(3) Council Tax (Administration & Enforcement) Regs 1992. The local authority may not apply for a liability order after 'the period of six years beginning with the day it becomes due'.
The 6 years runs from when the demand for payment is served on the taxpayer (Regentford v Thanet District Council  EWCH 246). Under reg19 the local authority should serve a demand on a liable person 'on or as soon as practicable after the day the billing authority first sets the amount of council tax for the relevant year'.
Advise them that if they wish to pursue this, that they must give you adequate notice, as you will wish to ask the magistrates to refuse to issue the liability order.