Yodel use self employed drivers, probably in a similar way to flex, but I doubt they get as much money - £30 to £50 a day - not sure how many parcels that entails.
I'd be interested to read an account of someone who has actually done this "job"* (*technically self-employed, therefore treated like "unemployed" to most financial institutions and don't receive normal benefits like holiday time).
Interesting that one got the mobile numbers of the customers to arrange delivery, while others clearly failed to deliver and then didnt get paid.
Yodel offer 50-85p per parcel: http://codforum.org.uk/forums/viewtopic.php?f=2&t=11911 Amazon Logistics in the UK: http://codforum.org.uk/forums/viewtopic.php?f=2&t=11936
There is also lifestylecouriers.net for anyone who wishes to delve deeper into Yodel, co (requires registration)