Accessing Numbers cell values from Swift using Scriptingbridge

I'm writing some routines to move data in and out of Numbers tables using Swift and Scriptingbridge. Most of it is working OK but I am struggling to read cell values which is kind of the point of the exercise. I can read and write other properties, like the cell's row/column, as expected but if I use:

var x = cell.value

I get a reply:

Scripting test[9307:928082] -[SBObject copyWithZone:]: unrecognized selector sent to instance 0x600000d15f50

If I try:

        cell?.value = "21"

I get the error:

Cannot assign to property: 'cell' is immutable'

I randomly tried:

        cell?.setValue!("21")

and it worked, the cell updated in the table. I tried:

cell?.getValue!()

which didn't throw an error but always returns nil. I tried:

        var cellObject = cell as! SBObject
        var x = cellObject.property(withCode: 0x4E4D4376)

(where 0x4E4D4376 is the AEKeyword for a Numbers cell value) which returns:

<SBObject @0x600000d176c0: value of <class 'NmCl'> 1 of cellRange of <class 'NmTb'> 0 of <class 'NmSh'> 1 of <class 'docu'> 0 of application "Numbers" (3184)>

which looks like the correct reference but there doesn't seem to be a way to get the actual value from that.

I can read and write all the other properties I've tried and used methods so something's Not Quite Right here. As it stands I can write to cells using a workaround but not read, does anyone know of any workarounds?

Thanks in advance!