Water pressure gone - but no filling loop to re-pressurise (help!)

Hi all

I am trying to get the water re-pressurised in our Baxi 624 boiler. I cannot get a plumber for a few days so I'm trying to see if I can fix it myself and would appreciate any tips during these cold times. 😐️

In summary:-

• water pressure on the Baxi boiler dropped to 0.5 two days ago, during the cold spell
• the gauge on the hot water tank in the hot press is at zero
• I cannot find a filling loop on the Baxi boiler, or in the utility room it's in, or under the kitchen sink, or at the hot water tank in the hot press upstairs or (from what I can see) on the cold water tank in the attic
• Baxi boilers seem to have filling loops underneath the boiler as standard but that's not the case here
• I think we have a gravity fed system, as there is a big cold water tank up in the attic. We only moved in a couple of months ago and the engineer's report didn't go into detail on any of this
• Baxi 624 boiler
• 3-bed semi-d, 20 year old house, 2 year old boiler

Many thanks for any help!!

Pictures below are of the kitchen sink (first), the hot press, and the last two are the boiler itself.