The starter solenoid is chattering because you haven't got enough volts & amps to trigger it properly, your battery is shot !
My S1 didn't have the removable bulkhead panel
Could you unbolt the relevant part of the SSL ? as that's how it's designed to wriggle it into place when being fitted
Turns out it's not the solenoid chattering it's in the dash so I suspect it's one of the relays as highlighted previously, all the gauges are dancing and the odometer flickering on and off, however even after what is supposed to be a full charge operating lights results in massive voltage drop, so the battery is goosed, I'm surprised it went from ok to that state in 20 minutes though, but the signs were there i.e. slow cranking.
One link is a short bushed one which I may be able to undo, however there is a welded bar that protrudes from the SSL frame across the top of the battery and bolts with a vertical strap holding the rear of the battery tray. There are two brackets at the front bottom of the tray which look like they can be accessed so allowing the whole tray to drop down, need to get it up on axle stands first though.
