There are aluminum replacements on eBay from Florida. I bought a set they are great. As stated on their eBay store:
*Some minor fabrication required as the old plastic roller needs to be ground off and the new aluminium unit install in place...
For automotive maintenance, always invest in a good clamp multimeter measuring both AC/DC amps & voltage, battery condition tester tool, a battery reconditioner/charger and a battery jump pack. ;)
Welcome to the forum.
Haynes only released a Disco 2 Td5 manual and no Thor V8 manual for us petrol heads.
As mentioned, the RAVE manual will be best to troubleshoot your Disco 2.