I'm Harvey the Master Plumber.
Your problem is: The Alpha SY24 boiler has stopped working. The pressure gauge
has dropped, you need to know how to re pressurise
once the radiators have been bled
Every hydronic heating system is required to have a presure regulator. Most guys call it a feeder valve. Every one I've installed or worked on had one. It's job is to keep the water pressure constant by adding water when you release some air without over filling the system. Sounds like it's not working. Some types can be serviced, others cannont so you may have to replace it. If you're not sure what to do it might be prudent to check the Repair Directory for a repairman.
Feel free contact me again!
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Harvey your Master Plumber