It's a rebadged boiler, not sure if Ideal or Glowworm...the case shape looks Glowworm.....but that's not important right now.
Fit the temporary filling link.
Pull the black lever on the right valve towards you to open it. If stuck take its handle off using a crosshead screwdriver, then use an adjustable spanner to turn the shaft anti-clockwise 1/4 turn when viewed from the right.
Open the other black handled valve to fill the system, close it when up to pressure.
Close the original valve before removing the link and re-attaching its lever.
vaillant who also own Glow worm, bought Heatline out, quite a few years ago now, they were just a cheap budget boiler, have seen worse boilers though, I used to be a repair agent for them and still have loads of stock which will never be used, hardly ever come across them these days, Vaillant were repairing them at one time but dont think they do anymore, not really sure though