Ordered a new plastimo tank which appears to be a direct sizewise replacement for the 30 year old melbourne made flexible tank in the boat.

Outlet will be a Fynspray vertical hand pump with 12mm. So I want run 12mm drinking water hose to the tap yes? 2 jubilee clips and done?

What's the cleanest way to set it up to fill it? Current setup is a bit of large diameter pipe (possibly 38mm) with a screw top lid on it that you just put the hosepipe in. I don't mind keeping a 10m drinking water hose in the locker on top of the tank for this job but it would be good to not prevent ever filling by decanting etc?

Thoughts please.

Yes it's likely 38mm. Smaller hose makes filling a slow tedious task. The larger diameter allows air to escape the tank as water level rises.

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.. Current setup is a bit of large diameter pipe (possibly 38mm) with a screw top lid on it that you just put the hosepipe in. I don't mind keeping a 10m drinking water hose in the locker on top of the tank for this job but it would be good to not prevent ever filling by decanting etc?

This is what I do.
A 38mm deck filler leads to my first 100 L bladder tank, easy peasy. Then I have another 100 L tank on the other side of the yacht. Unfortunately, I have a 12mm hose between these tanks. The 12mm pipe was easier to fit through the furniture, but also because each tank has a 38mm and a 12mm inlet/outlet. It is a bit slow transferring between the tanks. (One tank fast via the half metre of 38mm hose; the other tank is permanently connected via 12mm pipe, so a bit slow to top up 200L).
And I have a water filter at the end of the process, and an electric faucet.

I fill up via a hose, because there are public taps where I sail, plus marinas. The deck fitting also allows me to fill up using a 20L flexible tank which has handles (a bit like a jerry can, but squishable). This allows to top up when I don't want to moor the boat at a busy jetty, plus to top up if I get creek water via the jerry can and dinghy.
And a carbon filter just to ensure its clean.
And I keep a 20m flat hose and fittings always on board.

Phil do you have a 12mm tee between the tanks and the filter?

I think so. For the 2nd tank, the inlet is the 12mm from the other tank, and most probably a tee to the filter and up to the faucet.
That is easier than reducing the 38mm inlet/outlet, so it is probably blanked off.

First 100L tank is starboard side, next to the companionway. The second tank is port-side under the kitchen/galley area.