Where have I gone wrong?
Sorry to say, in a whole lot of places!
Bose-x has worked out the Pressure Altitude (from Sea Level)
No he hasn't, he has worked out the pressure altitude assuming the sea level pressure is 992mb, which it isn't!
I'm not quite sure why you would want to know the Aerodrome Pressure Altitude other than to say to yourself that today my aircraft performance will be 630ft worse today than a "standard" 1013 day
No, on a 1013 standard day, the pressure altitude at the aerodrome elevation (the 'Aerodrome Pressure Altitude') would be the same as the physical elevation, i.e. 968ft. On the day in question, the sea level pressure at 1024mb is higher than standard and so the pressure altitude is lower at only 630ft and, leaving aside temperature effects (we really want to know density altitude, of course), the aircraft performance will be improved not worsened!
so instead of the aerodrome being at 968ft on a "standard" day - today it now feels like it is 1,598 ft = therefore less performance
No, as above it actually feels like 630ft = better performance.
so why are Bose-x (1,598ft vs 968ft) and Islander2 (630ft vs 0ft) both showing worse performance.
Only bose-x is showing worse performance and that's because his calculation wrongly uses 992mb as the QNH rather than the QFE. As above, my calculation shows the pressure altitude to be lower than the actual altitude, with a consequent improvement in performance (assuming it isn't markedly hotter than standard).