Battery Nominal Capacity
With the information you added in the comments of your question, I'm pretty sure now that the charger you used killed the battery by overcharging it.

It says "Please note - Not suitable for AGM batteries" in its description and you've got a relatively low capacity AGM battery. The charger probably kept the battery at a voltage of more than 14V for the whole night, overcharging it dramatically.

Edit: The description of the charger also says that it's "not suitable for long-term connection", indicating that it does indeed just keep pumping current into the battery even when it's full.

You should use a 16Ah deep-cycle 12V battery with a suitable charger. You also shouldn't let the battery charge over night but stop the charging immediately when it's full. (Or get a charger that does this for you)