What is the Legal definition of a minibus?

Although minibuses come in different shapes, sizes and weights. The legal definition of a minibus is as follows:

"Minibus" - a motor vehicle which is constructed or adapted to carry more than 8 but not more than 16 seated passengers in addition to the driver.

(Construction and Use) regulations 1986

However, the above definition makes no mention about weight, and as a result minibuses could weigh anything from 2000-5500kgs gross vehicle weight or MAM (Maximum Authorised Mass)

However, volunteer minibus drivers who passed their car driving test after 1997 are restricted to driving minibuses that weigh no more than 3500kgs gross vehicle weight or MAM (Maximum Authorised Mass)