It turns out CTEK do a suitable 12v socket that comes with a replaceable standard spade fuse (which you may want to increase the size of) and will plug into the CTEK connector that is wired to the battery

https://www.amazon.co.uk/dp/B002X6DQV4/ref=pe_27063361_485629781_TE_item

And the description if the link doesn't work CTEK CTE-56573 Comfort Connect Cigarette Socket Adaptor - Black