UK Landlord Forum / Life

Does anyone know if a section 21 has to be a stand alone letter or wether it can be part of a longer letter?  Ive sent a letter to my tenant putting my opinions on some issues and in the last paragraph I've informed them that i wish them to vacate by virtue of section 21 etc.  They've told me their solicitor has said it isn't valid. Any guidance appreciated as the stress levels are rising rapidly!!!