radicalrabit,
My guess would be over 5,700kg, so you would need 2 type rated pilots.
Otherwise ...see schedule 8 ANO
http://www.opsi.gov.uk/si/si2005/20051970.htm#sch8
(1) Subject to paragraph (2), the holder of a Private Pilot's Licence (Aeroplanes) shall be entitled to fly as pilot in command or co-pilot of an aeroplane of any of the types or classes specified or otherwise falling within an aircraft rating included in the licence.