A flare nut spanner might be the answer. It is a type of open ended spanner with a bigger purchase. An example is here:
https://uk.rs-online.com/web/p/spanners/2358029?cm_mmc=UK-PLA-DS3A-_-bing-_-PLA_UK_EN_Catch+All-_-Mechanical,+Fluid+Power+%26+Tools-_-2358029&matchtype=e&pla-4574999185500285&cq_src=google_ads&cq_cmp=554644865&cq_term=&cq_plac=&cq_net=s&cq_plt=gp&msclkid=095f50e9815a1b0f8679389616c21e79&gclid=095f50e9815a1b0f8679389616c21e79&gclsrc=3p.ds&gad_source=7