Failing all of this, I guess I could get a 240v AC to 5v DC transformer
> and wire it in to the supply which is protected by a 3amp fuse. Would I
> need another fuse for this?
I've had great success with these...
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which you could splice into the mains., or alternatively
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is a simple 12V to 5V buck converter, that you could connect to the 12V battery