A small company over here has recently released a high efficiency 11W solar panel. They claim that it works well in low - mid sunlight. It looks good and am considering getting hold of one. The price seems reasonable if it does what it claims.
(Update 2/3/15 - 11W model no longer available but I did get hold of one and it works well!). The current range available is here ... http://www.portablepowersupplies.co.uk/solar-chargers/I have some other bits already from this company and so far they have all lived up to the claims on the site.