Setting Up A 'Temperate' Pond

I have been extremely interested in keeping temperate fish for a while now and having no space for another tank in the home I decided to construct a small heated mini-pond from wooden sleepers in my garden.
The climate is not too extreme where I am situated so I came up with the idea of expanding into the garden.
9 200 mm x 100 mm x 1200 mm sleepers were used for the basic structure which was then lined with 25mm insulation to prevent heat loss.
The capacity is approximately 300 litres with the filter.
Heating is provided by 2 x 300 watt titanium heaters which are connected
to an Inkbird Temperature controller.
My intention was to have some Shiners in the pond so I went for high flow with a large capacity pressure filter. The pump is rated at 4000 lph so I am guessing I am getting about 3000 lph based on the size of the tubing used etc.
I also wrapped the filter and pipes with thermal insulation to prevent heat loss during circulation.
The pond is covered with a clear plastic insulated sheet to prevent heat loss during the winter months this will be removed during the day when it gets a bit warmer.
The circulated water is currently returned to the pond just below the surface of the water. I also have a return above water level for use during the summer months.
I have attached some pictures of the pond during construction.

Setting up.
Construction was started in October 2017.
The pond was initially filled with tap water and water conditioner added.
I also used cycle and food to get the filter going.
Temperature is set to 19 centigrade with a 1 degree drop.
Added oxygenating plants (Elodea) and rocks with java moss and Anubias attached. These are all growing well.
I installed a Seneye device to monitor the temperature / ammonia levels etc.
During this month I changed a small percentage of the water using aquarium water from my tank on a daily basis (I still continue to do this).
This was left for a month after which I added 5 White Cloud Mountain Minnows from the temperate section of my LFS.
Ammonia levels went up slightly to about 0.02 ppm then dropped to 0.01 after about a week Currently the level is averaging 0.005 ppm.
The PH averages out to about 7.5.
I have been gradually stocking the pond since then and it now contains
8 x Rainbow Shiners, 6 x Red Shiners and 10 White Cloud Mountain Minnows.
I have also added a few nerite snails and there is a small population of MTS present.
Once the 2 x 300W heaters cut in it takes about 1 hour to increase the water temperature by 1 degree. Normal heating period is about 1-3 hours per day, however, during the recent cold spell the heating period increased to 4-5 hours per day.
All the inhabitants are doing fine. They are feeding well and spend most of their time towards the bottom of the pond darting about in the flow from the outlet pipe.
I have taken some underwater video footage using a cheap waterproof camera. It's not brilliant but at least I can see the fish in their environment. I will try to find a way to share this for your perusal.

Fantastic. 

Great idea to expand to a pond when you run out of room in the house.

Great fish as well. I have rainbow shiners in my river tank, as well as WCMM in my temperate tank. I don't have any red shiners, but I have seen them at an LFS. What a great mix of colours.

Thinking about the summer temperature problem. I could stick some silver tinted solar film onto the cover. This would reflect heat away from the water surface. Also a gap under the cover would allow air to flow across the water surface. The downside of this would be the effect on plant growth.
Alternatively I could stretch a mesh across the surface to stop predation and leave it open. This will allow me to cascade the water back into the pond via the filter which will assist with aeration and provide a cooling effect.

I’m on the South Coast in Portsmouth.  During the recent bad spell the temperature fell to about -3.0 and the heaters were on 5-6 times a day for about an hour. Temperature averaged about 18.2 C during the cold spell.
The heating cuts in at 17.6 C according to my Seneye. The Seneye is about -0.5 C out and I haven’t bothered to set the trim adjustment.
I’ll post a picture of the temperature graph from the Seneye for the last month to show how the heating fared during this period. I’ll try and stretch it a bit so it doesn’t look too cluttered.

That graph looks as if your heaters are coping well with current conditions.

I've had more struggles with keeping tanks cool during the summer. I have a few temperate tanks, and some are more of a problem than others. I've seen my axolotl tank go through a couple of warm summers, and eventually bought a chiller unit for them because they can die at temperatures =>24C.
Shade, air movement and water movement are all very helpful, as you've already considered. Before buying the chiller I also floated ice packs in the axolotl tank (sealed in ziploc bags, to be on the safe side), used fans at the top of the tank, and did large water changes daily, but it wasn't easy to keep >400L cold enough.
Considering the weather in the UK you probably have quite a while to implement suitable precautions before the warm weather arrives. 

Got the tag wrong on the last post. Hopefully this one will work :- #Invalid YouTube Link include https#

The link to my video, hopefully this one works :-
https://youtu.be/ksHuxrgA0Ok

Those are some active & perky fish.
Adorable. 

Wow - lovely. What voracious feeders they are! 

Sounds good!

Just a thought 're the gravel though... if its darker than the current base it may hinder your view of the fish... it would make maintenance less easy too...  that said it will likely enhance the look greatly! Can you see the base from the surface?