Chris Plastow
It will not be residual gas . It may be since other gas works were carried out two years ago but without knowing what was done , if a new protective sleave was laid to your property , smell can be traveling down the sleave , this can blocked with foam. What I suspect has happened is , the normal flow of air under your house has been blocked in some way , now the air escapes through your cupboard not the external air vents , check for problems here. One can airtight seal the floor of the cupboard if one likes .
