do they have to be a certain size or are they e marked, was just going to go to halfy's

SVA manual (online version) states the following about Rear Reflectors:-

Twe rear reflectors, red in colour, not more than 400mm from the side of the vehicle, And not less than 600mm apart (400mm if vehicle is less than 1300mm wide) Maximum height 900mm (1200mm if impracticlable), minimum height 350mm. They must not be triangular in shape. Approval marks E, e, BS 1 or 1A.

HTH

They don't have to be marked - if they're not they just have to operate similarly to if they were - difficult to test I would think, so they're unlikely to fail provided they're sensibly sized.

Kingr