I currently have a Worcester Bosch (WB) 20/25 Heat Slave Combi (non condensing) oil boiler dated 1999. I emailed WB and they said they don't have accurate figures for it's efficiency as it's pre-SEDBUK, but it's probably 65-75%. I'm getting, I think, a WB Heatslave II 18/25, which has a SEDBUK...