Historic England has recently published "Gas Works and Redundant Gas Holders: Guidelines for their Evaluation and Recording". It is available to download here.
The guidelines are designed to inform an understanding of the significance of gas works and gas holders, whilst ensuring adequate records (both above and below-ground) are made where evidence is likely to be damaged or destroyed. Such information, gathered in a proportionate manner determined by the significance and characteristics of each site, may in some circumstances guide decisions regarding the possible retention or inclusion of historic fabric and site layout within a future redevelopment. Such an approach is also of relevance for buildings and structures, including gasholders that are to be conserved, including those where dismantling for repair may be necessary. The guidelines also take into account the important role of community engagement and oral history projects in capturing the memories and experiences of those who worked in and lived adjacent to gasworks.