Setting Up A 'Temperate' Pond

Lovely little fish for sure...  I can't really advise on them myself, as I know virtually nothing about them, but can completely understand wanting them...  Hope they are suitable, coz they're great l'il fish...   

@Matt  yes, I've got a group of sewellia lineolata in my river tank.

@daveyng there are a few types available, which were you considering getting? They do like a lot of flow, and need high levels of oxygen in the water. They may also appreciate some larger rocks among your pebbles.
My gang enjoy algae & biofilm, but are also happy with the Repashy food, algae wafers, cucumber, etc.

They are lovely looking fish. 
They are wild caught and quite sensitive, so make sure that they are feeding at your lfs prior to purchase.
Although linear water flow is ideal, it is not absolutely essential. Cool water and high oxygen levels are essential. You may want to look at how you are going to keep the pond cool during the summer, and how you are going to deal with surface evaporation (may need a solid lid, and top up losses with RO).
I have a group of 9 in my river tank, and they seem fine together. I got quite a few because SF says they are found in large groups in the wild http://www.seriouslyfish.com/species/sewellia-lineolata/
Although they are said to be relatively aggressive, my gang seem ok as long as each one has a large rock to themselves. They mix well at food time, and mix well with my other fish. When I first got them two of the males had a battle for the best rock, but that was short lived with no harm done (and fascinating to watch).
As for the behaviour of your fish, things might change slightly as your plants grow. However, the rainbow shiners in my river tank are usually low to mid-water, rather than near the top, which seems to be the same as what you are seeing.

... I use a small one on my aquarium and it seems to work.

Wouldn't mind seeing a pic of that Dave...  Looks like we're in for a hot summer and cooling the tank is likely to be a challenge...

@TopCookie there’s a pic of the fan under the topic ‘New Lighting AI Hydra 26 HD’ you can see it behind the lighting unit. It’s fairly small but it works for me.

That sounds interesting. Let me know how that goes as I may need something for my river tank that is cheaper than a chiller. Obviously as my tank is indoors things are a lot easier for me, so I'm going with fans & water changes at the moment. 

Latest Readings from the Seneye in the Pond (attached). You can see the temperature is creeping up. The pH had been fluctuating a bit but I managed to curb that by covering the Pond with a sheet of the mirrored insulation during the peak sunlight hours (13:00 - 17:00). Today its gone up because I ran the water feature. Probably drove the O2 level up.
There are also a few more pics of the setup (bit Heath Robinson) but it works. There are 2 showing the cover. I place a couple of rocks on it to stop it blowing away !!. There is another of the filter with the insulation wrapped around it and 1 of the working water pump feature.
I am going to construct a rustic looking box to cover the filter although its tucked away and not really noticeable. I will also do something more permanent for the cover.

Back on topic now:  love that vintage water pump set up - that looks fantastic...!!!   

I did consider the standard "PC" fans in a case, which I have on top of my tank (see pic). However, I think the Hydroponic / Bathroom inline one would be ideal, there much more powerful. I could simply cut a hole in the cover attach a 100mm hose to it via a standard fitting and then attach this to the fan.

Great to hear that all your fish have settled in and are doing well.