Vailliant EcoTEC Pro - temperature dropping really fast

[Frandamby]

Hi all,

I’ve been tinkering away today as we have noticed our radiators are not giving out as much heat as they once did. Our house is so so cold.
We have 1 radiator in our bedroom which gets stinking hot though, the rest are no where near as strong.

I’ve done a few things today and they haven’t helped.
1. Bled the radiators to reduce the pressure (it was 3.0 bar and now down to 1.5 bar)
2. Range rated the boiler. Set the KW to 20kw (not sure of the original setting).
3. Increased the temperature on the boiler (water to 60 and CH to 70)

What seems to happen is the boiler starts up, the temp rises to about 55 and then boom, orange light under the temp display goes off and temp drops really fast back to 40 degrees. I then get a S.7 code (not automatically, only if I check). Something to do with the pump over-run?
Whilst the temp is rising, the CH flow does get hotter then it cools when the temp drops. The CH return pretty much stays the same.

I’ve attached a few picture to show my pipes and boiler which might help!
I’m googled out guys - any advice?

Thanks
Fran