From what I can tell, unless insulin-treated there's no automatic bar to a diabetic driving a minibus, although if insulin-treated you would be classed under the Group 2 standards (ie the same as an HGV). The Group 1 standards (ie private cars) are more lenient as regards insulin.
Have a look at the document which is linked to towards the bottom of the link below, which covers the issue chapter and verse and there's a whole chapter on diabetes.
It's called 'At a glance', but I think they must be taking the Mick :lol:
http://www.dft.gov.uk/dvla/medical/ataglance.aspx