How can I fix the timer on my Gledhill Boilermate central heating boiler?

I have a Gledhill BoilerMate system driving my central heating system which has a Grasslin Thermostat timing system.

I noticed recently that the central timer part is not physically connected to the outside casing resulting in the central timer part falling inside the cabinet. For the moment I have partially jammed it with a rubber washer. However, this is clearly not a safe situation I suspect as the timer keeps falling out of the casing back into the cabinet! I’ve attached a photo of the timer jammed into the casing.

From what I can see there are four connecting screw holes two of which don’t appear to have holes all the way through but two do? One of the screw holes appears to have a screw on a spring but it doesn’t go completely through to the matching hole at the rear of the timer and the screw head appears to be partially broken? The other screw hole appears to have a thin prong in it. It appears that two of the holes on the casing match up to two holes at the rear of the times so presumably can (or should be) screwed together?

Can you please send advise exactly how the timer should be connected to the front casing such that it doesn’t fall back into the cabinet? Assuming that it should be screwed together I’d appreciate if you can advise what type of screws are required and where I should be able to get them if they are a special screw type as I don’t want to use the wrong type of screw and break the holes at the rear of the timer?