I need a bit more ventilation in my loft without doing anything which will
require building regs. The enclosed area is about 12 x 14 ft, and is in use
for a model railway room. Local inspectors have frowned on similar usage
near by and declared it to be habitable. Are there any window products or
vents which can be fitted between the rafters without cutting into the
structure (i.e. removeable if we sell the house). The rafters are at nominal
16 inch centres (they vary by as much as an inch.)

TIA

you could fit ventilated soffit boards and ducting into the room space.