technically it is a health & safety executive matter.

you have to request it in writing. if the landlord doesnt do it you have to report them to the HSE and they prosecute.

because people dont tend to report their landlords (you want to keep living there, dont you) it doesnt happen that often. When it finally kills someone the HSE ask for the certificates, the landlord cant produce them and they get prosecuted then. Bit late for the person it killed, but this is they way our laws are slanted.