Its a tough one. I was same as you ten years or do ago did plenty of bathrooms worked on heating systems etc.
I went the college route 3 years most of it was basic stuff. Whilst at college i built a gas portfolio up which qualified me to take my acs.
Ten years on.......
I earn more money on...
If the joint on the branch was outside the externsl wall then the soil pipe would be stood off the wall too far.
The branch joint is always just inside the wall.
First pic not sure what thedark ring is round the pipe? Its either not fully home and its the bur or its a smear if silicone grease...
The brown and blue wires in your pics are common and heating on.
Trace them back to boiler or wiring centre.
You have posted the diagrams up daniel thats all you need
The receiver needs wiring near the boiler.
That way you can pick up live, neutral and earth up at the fuse spur.
The two wires in your existing stat will go back to the boiler.
These wires need to go from the boiler terminals to 1 and 3 on the hive reciever
Not sure but your probably best employing a heating engineer who is confident working on a s plan.
He will be able to trace the fault with a multimeter