Hey Nick,
the advice on bolle glasses is pretty god, the glasses work brilliantly for the money and do a fantastic job. What you mention about double vision/blurriness/distortion in cheaper glasses is really true. It comes down to the layers of the glass and how they bounce the light within the lens.
Specialized are actually the worst for this, so would advise away from them if possible. Sorry Del. The adaptive lens reacts brilliant, but the actual visuals make you feel a little drunk!
There are reasons that companies like Nike stopped making glasses, it's because it's a hard market to crack that requires a lot of investment. I mean, look at oakley, they almost own the market and nobody can come near to them in terms of quality.
I hate the company and their dominance on the market, but everything from their XYZ (retina profile corrected lenses (100% accurate vision)) to the fact that the lenses are not layered, they are a selection of plastics and films that are subjected t extreme heat, until they liquify and combine one uninterrupted lens is amazing.
As a comparison, Specialized adaptive lenses can have up to 7 layers of plastics, films and glue between your eyes and what you see, so imagine the distortion.
If you can find the money then oakley's will allow you to get out on the trail and will also you will save yourself a lot of anguish and time than buying and replacing cheaper glasses.
But, for £50 I would go for the Bolle glasses.
Thanks