A shadow over No 42

I am bereft of heat at present and winter is casting a long shadow over No 42.
My feckin’ heating has packed up. It’s an Alpha CD25C/28C/35C wall mounted system and a friend suggests “Usually a loop (small pipe) not connected to system. It has to be connected to re-pressurise the system. Actually a water feed. Some systems the loop is internal in which case plumber required. Some systems you can actually see the flexi pipe next to where it has to be connected. What a pain to have this happen now. Presume pressure below 1bar.”
Another pal suggests something about turning on the water via the inlet gap and says it’s easy providing I stop before the red line.
A link to a Utube video would be useful if any of you plumber folk oot there can help.
There’s a cake on offer.