AGA back boiler

[samol]

Hi,

I have a gas fuelled AGA we only really use during the winter. I have a back boiler for it which is currently not installed.
Our water heating system is quite elaberate in order to accommodate this back boiler, basically a water tank & gas boiler on a timer which isn't required as much when the AGA boiler is in situ.
When connected we had loads of hot water, but I disconnected it a few years ago because I was lead to believe that it was quite inefficient way of heating the water. But with it disconnected we quite often don't have enough hot water.

My question is, are AGA back boilers a really inefficient way of heating our water?

Thanks

Ant