I've retrofitted them myself and while they are excellent for controlling the comfort level of heating you want I''m not sure that you'll notice a huge drop in oil/ gas consumption over the short term. To maximise on the benefits of fitting TRVs you really need to bump up the insulation levels in your house, as a minimum you need 300mm of mineral/ glass fibre in the attic space, in addition to the usual draught sealing to minimise air leakage.