Five degree differential btw CH outflow/return - condensing combi

[Chalked]

Get those rads properly balanced, if captain plumb put inhibitor into the system then it could have reacted and left you with air in the system so make sure the rads are bled. After this i'd take a look at your pump, turning it can help!

Also you're right about the condense.. you want it around 55 if you want to save some cash.
The 70 degrees sound a bit too high!

The problem with condensing boilers is that the boiler, to run at its most efficient, likes to run at 50-60 degrees. But older systems were designed with a higher flow temperature. Therefore, need 70-80 degrees.
Best thing in an older sysem, is to have new oversized rads, but this costs and causes disturbance to the decoration.
So a compromise is to balance the system, ie. Restrict the lockshield to get about 15 degree difference between flow and return. Tune the pump setting to suit the system .
Then try running the boiler at 60. If the house becomes noticeably cooler turn up a few degrees at a time.