Is it being used for commercial purposes? If it's not then I think you're ok to not have one but check with VOSA. I know our work lorries have them but they are both 10 tonnes....
on a more than 3.5 tonne up to7.5 tonne you only need to use it if you are using the lorry for hire or reward as part of a business ie you transport horses , have a commercial stud, show horses for money, or are a horse dealer etc ... if its for your private leisure puposes then no...