This is what you need:
Specialized BG footbeds
Next please!
If the shoes are too narrow then footbeds are not going to solve it and may make it worse (the increase of thickness of inner sole).
Let me guess Tyres23, you've got Shimano shoes? Shimano shoes generally are too narrow. Always get a size up from normal if not two sizes up.
I get this on my right foot (right foot being slightly bigger) after about 45 miles in my Shimano shoes (RT81). I realised that after loosening the straps to the fullest extent, that the narrowness of the shoe is the issue. I don't get this issue at all with my Specialized shoes. Possibly being leather they have 'molded' to the shape my feet. I thought this may happen to the Shimano's but realised that they are synthetic uppers.
Edit: If your shoes are marginally too small then you may get away with just loosening them.