Citing an ISO standard with multiple parts, using Harvard style?
After speaking to university librarians about this and although they could not find any official guidelines, they suggested that within the Bibliography I reference each part of the ISO individually:

@techreport{ISO14230part1,
type = {Standard},
key = {ISO 14230-1},
year = {2012},
title = {{Road vehicles. Diagnostic communication over K-Line (DoK-Line). Physical layer}},
publisher = {British Standards Institution},
institution = {International Organization for Standardization},
note = {Available from: \url{https://bsol.bsigroup.com/en/}}
}

@techreport{ISO14230part2,
type = {Standard},
key = {ISO 14230-2},
year = {2013},
title = {{Road vehicles. Diagnostic communication over K-Line (DoK-Line). Data link layer}},
publisher = {British Standards Institution},
institution = {International Organization for Standardization},
note = {Available from: \url{https://bsol.bsigroup.com/en/}}
}

@techreport{ISO14230part3,
type = {Standard},
key = {ISO 14230-3},
year = {1999},
title = {{Road vehicles. Diagnostic systems. Keyword Protocol 2000. Application layer}},
publisher = {British Standards Institution},
institution = {International Organization for Standardization},
note = {Available from: \url{https://bsol.bsigroup.com/en/}}
}

@techreport{ISO14230part4,
type = {Standard},
key = {ISO 14230-4},
year = {2000},
title = {{Road vehicles. Diagnostic systems. Keyword Protocol 2000. Requirements for emission related systems}},
publisher = {British Standards Institution},
institution = {International Organization for Standardization},
note = {Available from: \url{https://bsol.bsigroup.com/en/}}
}


and then within the text to reference using the years in brackets: ISO 14230 (2013; 2012; 2000; 1999). Noting that even though the years do not correspond with the order of the different parts but when referencing works by the same author the most recent comes first.