Funny you should post this - five minutes before I read it, I'd been leafing through today's MCN and there was a mention of Optimate's Duo solar charger, which they specifically said can charge both standard and lithium batteries. Not cheap though.
https://www.optimate.co.uk/products/optimate-solar
I see Optimate do just sell the controller, which presumably could be used with your existing solar panel.
https://www.optimate.co.uk/products/optimate-solar-duo