Dave if its the same type as mine, its a ring ultra with a external switch.
You have to gently squeeze the top and bottom part of the outer casing to release the four clips that hold the outer cover in position, and pull the cover towards you.
The bulbs for me are 12volt 10 Amps(Max)qty 2.
When replacing the cover make sure the clips are back in place by gently tap the edge of the cover with your hand.
Royston