Found this on another forum,
Fixing of front registration plates: vehicles registered on or after the relevant date
6. - (1) This regulation applies to vehicles, other than works trucks, road rollers and agricultural machines, first registered on or after the relevant date.
(2) Except as provided in paragraph (5), a registration plate must be fixed on the front of the vehicle in the manner required by paragraph (3).
(3) This paragraph requires the plate to be fixed -
(a) vertically or, where that is not reasonably practicable, in a position as close to the vertical as is reasonably practicable,
(b) in such a position that in normal daylight the characters of the registration mark are easily distinguishable from every part of a relevant area having the diagonal length specified in paragraph (4).
(4) The diagonal length of the relevant area is -
(a) in the case of a mark having characters the width of which is at least 57 millimetres, 22 metres,
(b) in the case of a mark having characters the width of which is 50 millimetres, 21.5 metres,
(c) in the case of a mark having characters the width of which is 44 millimetres, 18 metres.
(5) In the case of a motor cycle or a motor tricycle which does not have a body of a type which is characteristic of the body of a four-wheeled vehicle -
(a) a registration plate must not be fixed on the front of a vehicle if it was first registered on or after 1st September 2001,
(b) a plate need not be fixed on the front of the vehicle if it was first registered before 1st September 2001.
Fixing of registration plates: vehicles registered before the relevant date
7. - (1) This regulation applies to vehicles, other than works trucks, road rollers and agricultural machines, first registered before the relevant date.
(2) Except as provided in paragraph (4), a registration plate must be fixed in the manner required by paragraph (3) on -
(a) the front of the vehicle, and
(b) the rear of -
(i) the vehicle or,
(ii) where the vehicle is towing a trailer, the trailer or,
(iii) where the vehicle is towing more than one trailer, the rearmost trailer.
(3) This paragraph requires each plate to be fixed -
(a) in a vertical position or, where that is not possible, in a position as close to the vertical as is reasonably practicable, and
(b) in such a position that in normal daylight the characters of the registration mark are easily distinguishable, in the case of a plate fixed on the front of the vehicle, from in front of the vehicle and, in the case of a plate fixed on the rear of the vehicle or trailer, from behind the vehicle or trailer.
(4) In the case of a motor cycle and a motor tricycle which does not have a body of a type which is characteristic of the body of a four-wheeled vehicle, a registration plate need not be fixed on the front of the vehicle