For anyone needing to replace a lower balljoint on an earlier 5EXI, I believe this is the correct part:
http://www.ebay.co.uk/itm/Vauxhall-C...item3cbf929cdd
Front Vauxhall Cavalier Mk1 1.9 OHV RWD Oct_1975>Mar_1978 Ball Joint - Upper RH - Part No: FBJ5045