It is more expensive to top up your allowance but I believe this is because Zen purchase DSL transfer (from their network to your home or business connection) in bulk from BT each month based on a projection of what their body of customers will use based on the packages they are on.
When you top up they have to purchase additional transfer from BT which maybe charged slightly higher and they do of course have to make a profit on it.
Another thing to factor in here is the additional charges they may have to pay to their transit providers.
This is a complete guess and I'm sure someone from Zen will be along to explain.
I upped my package from Lite to Active this month for this reason as it was cheaper to commit to the additional transfer than to buy it on a prepaid basis.
So in a nutshell I think not
Virgin (ADSL) => Namesco => Newnet => O2 => Plusnet => Zen => Newnet => Zen Lite 8000
Note: I don't lay turf for anyone. astro or otherwise, all views and opinions expressed are my own based on experience.
Edited by techguy (Sun 02-Sep-12 18:59:10)