malcolmp

Yesterday I removed the new inside tap, and the flexi-hose, and put a temp used tap to test the water, we noticed that the water tasted much better.
I then soaked those in Bicarbonate of Soda for a few hours and then put them back. It did seem much better. Then we thought we should run Bicarbonate of Soda diluted with 25l of water (750mg Bicarb) through the entire system, which we did about three times - so almost 75l ran through the entire system.
Since we did this, the water no longer has the metal taste. My assumption is that it was either the tap or the flexi-hose. 
If things change I will update this post, but for now I have a good feeling our problem is solved.
Remember the only things that changed was the pump, the flexi-hose and the tap.
Thanks for all the input, it is always appreciated. 

That would seem logical at first but keep in mind that air is 800X less dense than water so even a leak that causes a tiny drop of water to come out would let in over 800X more air by volume when sucking. Density of air - Wikipedia
https://en.wikipedia.org/wiki/Density_of_air#:~:text=At%20101.325%20kPa%20(abs)%20and%2015%20°C%20(59,International%20Standard%20Atmosphere%20(ISA).

Sounds OK that air can be sucked in but less of a problem as atmospheric pressure is pressing water into the pump due to gravity feed although there is still suction. 
This is what I found and I work on the sound from the pump to know when my system is sucking in air when the inlet side tank is nearly empty. 
Just how I think about it. 

Seems I am also a shoplifter!

These forums are great to get advice and see how others have solved issues. A big thanks to all for your input. And here is the solution.....
I went through the steps advised - even though I had done most of the steps before... Removed the outlet and turned on the pump - water trickled out as before. No pump on just water passing through the pump - barely anything coming out. Then.....
Removed the inlet pipe from the pump - hardly any water coming out. Removed the 25mm pipe from the 44 gallon drum - water gushing out - put 1 times stopper in the drum outlet. Tried to blow air through the inlet pipe and could hardly get any air going through. Start removing section by section, and in the one elbow, some tosser at the shop had pushed a stopped into the elbow, which I did not notice when assembling. Removed that stopper and put everything back and the pump works perfectly, singing along with full pumping sound.  
So Ardendorf service customer care was useless, as they told me there was not enough pressure coming from any 44 gallon drum, and that it had to be the size of 1000l Jojos to be able to work. 
The short of it....check joins before assembling your pipes.
Some pictures attached for you guys to chuckle about.🤣

Stupid question.
Did you bleed the air out of the pump?

I would disconnect your outlet pipe and see how it runs with the 20/25mm diameter outlet un-restricted. If the flow is still very weak then disconnect the inlet and see if the flow from the jojo is what you expect. If all appears normal then your controller is the likely culprit, you can replace the internal pcb for about R250 if you have trouble with the supplier re warranty.
FYI : The controller detects pressure drop to turn on pump. Once pressure restores to pumps spec. i.e. 4bar, the pumps internal switch will turn it off.

I agree with @Scorp007 there is a mechanical noise that does not sound right. I run quite a few similar pump here on the farm and they are very quiet you only hear a slight woosh sound and can actually hear the water running through the pipes. I would investigate further before running the pump as is. Remove the back fan cover then start the pump and see if the noise clears.

Sorry I missed the part that you changed to lithiums. I would stick to the 100A for each battery. 

Of course these inverters work well with Li batteries. Its a matter of setting the correct bulk and floating voltage. And 200Ah for 5kW inverter seems to me rather low. Remember, the lower the battery capacity the higher the stress on the battery due to heavy load.
I have 2 Axpert/Synergj MKS5K in parallel with 4 packs of 100Ah 48V LiFePh, that's a total of some 19kWh. They run now for more than 2½ years with over 800 cycles. SOH still 100%. It's better to get 48V packs with BMS as these are industry standards. The ones I have can be rack mounted but I installed them upright on the floor.