The MDL ones are "coded" and, allegedly, "recognised" by the supply sytem so that you can just plug it into any of their marinas and the bill still comes to you. We have one and the system seems to work well. However whether a non-MDL supplied lead would be "recognised" and accepted is doubtful IMHO.
I too was not a happy bunny at having to pay a mark up on Marina prices for a shore power lead, so i rang Metermaid direct and they quoted exactly the same price as the Marina. It would appear they are supporting the monopoly they have, although they tried to explain the difference as being due to economies of scale, the marina's buying so many at a time they get a big discount.
In the end i had to pay the marked up price £150 if i remember rightly. If you do manage to find another supplier willing to deal direct you will then have the problem of the marina accepting the certification of that brand before you can connect. SNOOKERED.
The MDL leads don't need to be coded or need to recognise anything, they just need to be meters. If you use it at another MDL marina it just keeps the meter running so that next time your meter is read at your home marina it includes the usage at the visited marina.
This would also mean that if you use it at another non MDL marina where you are paying for electricity it would also run your meter on and this is also likely to be charged again by MDL when it is read later, so I would either double check or be careful.