I see parts listed by VIN eg this service kit is for V8 EFi Petrol models to VIN TA346794 and WA376580 to 385948 https://rimmerbros.com/Item--i-RA1479
While this is for VIN VA346795 to 376579 & WA385949 https://rimmerbros.com/Item--i-RA1480

My VIN is SALLPAMJ31A458723 which doesn't seem to map on to the VINs listed. How do I decode it? Is it just the six numerals at the end that matter?

yes I believe you are right it is the six numerals at the end

The letters at the beginning of the quoted VIN numbers are the model year, T being 96 (as although production started in 95, they were all 96 models), U was jumped, V being 97, W is 98, X is 99, Y is 2000 and then they started at the beginning (as Z would be too easily misread as a 2) so 1 is 2001.

GEMS models were up to WA410481 with Thor starting at XA410482 (so the year designator changed but the serial number carried on). While the actual numbers show the particular serial number for the car but that doesn't mean yours is the 458,723rd P38 as these numbers continued on from the Classic which used the same format

The rest of the VIN gives detail on the build, so yours shows it was built in the UK by Land Rover at Solihull, it's a P38 with a 4.6 litre V8 engine, a ZF 4 speed auto gearbox and is RHD.

The VINs quoted ie GEMS up to WA410481 & Thor starting at XA410482 have the format of two letters & six numbers while the last eight digits of my VIN are 1A458723.

Sorry, typo in my previous (now corrected), Y shows 2000 but then they ran out of alphabet so went to numbers from then on so the 1 means 2001, A means built at Solihull and 458723 is the serial number of the car.