Minimum flue size

Boxing in twin wall flue pipe - in Document J, page 21

Stove building regs pages:
Stove building regulations intro
Notice plates
CO Alarm
Installation and Building Control
Stove hearth sizes
Flue outlet heights
Flue sizes
Ventilation for stoves
Flue pipe - distance to combustibles and heat shielding
Flue pipe - recommended length
Twin wall flue - boxing in
Lining existing chimneys
New chimneys

When do you have to box in twin wall flue pipe

When twin wall flue passes through a cupboard, storage space or roof space a guard should be placed that is not closer than 50mm to the flue (well actually the distance to combustibles as defined in BS 4543-1 if we are being pedantic).
The most normal and easy way of achieving this guard is to box in the flue pipe. Only Selkirk STC twin wall flue have dealt with the safety issues when boxing in twin wall flue pipe - read more here.