What is "SEDBUK"? 

SEDBUK stands for "Seasonal Efficiency of Domestic Boilers in the UK", and is an industry standard for measuring and publishing the thermal efficiency of a boiler. This is so the public can compare the economy of various makes and models of boiler on the same fair basis.

They have a website www.sedbuk.com, where you can look up your own boiler, or the one you may be considering buying. Lots of other interesting stuff (interesting to plumbers, that is!) about boilers there too. Highly recommended by me.