Hello LostInSpace.  I'm also fairly new to this but will attempt the usual introductory run down of necessary information.

For starters, what controller do you have and where is it?  You should have installation documentation about the various aspects of the system. Also how big is your house, 16kW implies a somewhat bigger house than average, so higher consumption is expected.

There should also be heat caluclations, at least very rough ones, from the quote, which should give us an idea of what the installers were expecting.

Lots of people on here can discuss flow rates and weather compensation settings, but until you can see them yourself on the system it is not as clear.

Also for comparison we have a 12kW system and at the weekend with -4C at night and 0 to +2 in the day we used about 34kWhr on the heating and hot water, so it does sound to me like things are not optimised. 

And by turned the heating on upstairs, was the system specified to run upstairs and downstairs heating at the same time?  So in turning it off you have halved the required flow volume and heat from the heat pump?

I also have a 16 kw system and I am shocked at the amount of electricity that is required. Yesterday in 24 hours we used 57 kw and we are aiming at 19c. We have an Ecodan system  which has a low COP-we get just over 3 and that is using a compensation curve and running it 24 hours as recommended.

it is much more expensive to run than the oil boiler.

from midnight to 9 am the house used £9 of electricity. Most of that is the ASP.  Apart from a fridge/ freezer nothing else was on.

i don’t have solar panels so I’m stuck with a system that is burning up our pensions. 

I have a 16kw samsung. yesterday with outside temps stuck around zero all day, I used 45kwh of electricity, for 120kwh of heat (including quite a lot of hot water). that kept the house at 21C 24/7.  thats by far the worst day so far this winter. most of the time running at 2 to 2.5kw of electrical input, for 7 to 8kw of heat output, thats including defrosts. today more or less  the same. at 5C outside I get way better. 

the vast majority of performance issues are down to the overall system design, not the heat pump itself.

To start with, please just try to describe your system. Some specific things to cover:

what controls are you using? an external thermostat or is the samsung controller being your thermostat? Do you have Weather compensation (Water law) enabled or are you using a fixed flow temperature? Do you have a buffer tank or a low loss header? do you have TRV's (or the UFH equivalent) or are the zones open all the time? what do the flow rate and water outlet temperature say when you go into the diagnostics on the controller?

"turning the heating on upstairs" may actually be part of what you need to do. To cut to the chase (there are many in depth explanations of this but please trust me) : heat pumps will always work better, the more emitters you can get in the circuit, fully open to constant flow. its a difficult one to get into your head if you are new to a heat pump, it certainly was for me.