Hello Rediac,
I'm not familiar with Abbey Vogue light units nor have I seen the handbook,I can only go from your descriptions.
First of all it's unusual for the light unit securing screws outside the light lense,so I'm finding difficulty picturing your description of having the light unit in your hand.
That aside,if there are no securing screws holding the lense onto the back plate then I can only suggest the lense is a press fit.Try gently pressing the top and bottom of the lense unit and extracting the lense unit from the back plate.
Let me know how you get on.