Hi mate .. been out welding all afternoon.

Right .. yes, I can see the fill loop in that pic : ) Obviously you sussed how to top up the pressure then.

Over the course of a few days use .. keep bleeding/topping up pressure until it (hopefully) stops dropping.
You will find that as you get most of the air out, the pressure comes up quickly ... only needing a few squirts.

Leave the rads furthest away from (most likely slowest ones to heat up) open more on the lockshields than the near ones.

When the system is balanced, another bonus is that you can knock the pump speed down to possibly minimum .. depending on system volume and circuit length. This will save on lecky, too .. and is quieter in operation!

Remember .. the thing you want to get is that 18-20C drop across the rads if poss.

SYL