[MTBRider]

Thank you all for the detailed responses. I think there's a strong possibility i am over thinking it.

The dual input MPPTs look like exactly what I want + hookup.

The only reason I wanted the mains hookup to have priority over solar, is that, the only reason I want to use mains for charging / conditioning the batteries. The method that @Hoovie suggested sounds like it will do the job perfectly.

@dobris I have been looking at Victron components over the coming days.

My intention now, is to complete a wiring diagram, which I'll post here for your comment and then spec all the components too fit. I'll likely build this out in a modular steps due to the cost. I.e leisure batteries + alternator charge first, then solar, then hookup. I'm a little bit of a control freak / perfectionist on these things.

 

[MTBRider]

doing stuff in stages due to budget makes sense but .... Do make sure you get all your cabling in place ready e.g. all the "first fix" stuff in house building terms.
For example, a common place to have a hookup plug in a T5 in under the bonnet. If you do this, then you will have a mains lead from Engine bay to cab. This will follow the exact same run as the Split-charge cable from Engine bay to cab, so it makes sense to run them at the same time.
You can add in the actual plugs and consumer units later on.

If you are a bit of a control freak you want to be able to see what is happening with the electrics probably (hence why you were thinking of the Aduino approach?). To scratch that particular itch you could (and are anyway) look at Victron kit which has probably the most advanced connectivity of the mainstream products?
As an example, I just logged onto my Campervan whilst sat at my desk in the house and took these two screenshots ...

A view of some of the devices (so Fridge is healthy at 3C, the Heaters fuel tank has used 1.436L since I last filled it up, the Charger is in Storage mode, etc)
View attachment 175904


And a summary of the charge - and energy production and use - of the system
View attachment 175905

I may be a bit of a control freak as well

That was exactly my plan.

I really like the look of the Victron stuff. The connectivity looks awesome too. I'll work on planning out the system.

 

[MTBRider]

So if you used this wiring set up...

How to Wire up your Camper

Wiring up your camper van can be very daunting at first, however if you follow some simple rules, it can be just as easy as wiring up a normal household plug. Below is a diagram that shows a typical...

www.switchpanel.co.uk

Could you just add an extra input to your leisure battery from an mppt controller for solar? If that makes sense?

If I have understood @Hoovie correctly, the MPPT will have two inputs, one from VSR and one from Solar. But otherwise i believe that circuit matches my intentions.