This paper describes the design, construction and performance checks of NPL's upgraded High Temperature Guarded Hot Plate (HTGHP) carried out following the guidelines given in ISO 8302:1991, EN 12667:2001 and prCEN/TS 15548-1:2007. This upgraded HTGHP is designed for thermal conductivity measurement of insulation and refractories in the temperature range 140 °C to 800 °C, having thermal conductivities of up to 0.5 W/m·K. It is a single-specimen apparatus that accommodates flat-slab specimens of diameter 305 mm and thickness from 25 mm to 60 mm.Design and construction information is presented for: (1) a main heater plate and a heated cold-plate, including the arrangement of thermocouples, thermopiles, heaters, determination of the heater length in the metering area using X-ray photography technique, and high temperature high emissivity paint; (2) a guarding system, including an auxiliary guard plate, a lower edge guard, and an upper edge guard with a similar temperature gradient as that on the specimen; (3) and an in-situ thickness measurement system.The performance of the upgraded HTGHP conforms to the requirements given in the international and European standards, and the measurement uncertainty is improved to ±4.0 % with a confidence limit of 95 % over the whole temperature range. The comparison of the results from proven performance check on a NPL internal audit specimen, with values measured using the previous high temperature guarded hot plate at the NPL, showed agreement to within 5 % at temperatures below 600 °C. But the difference increased as the mean specimen temperature increased, e.g. at 700 °C the difference is 6.5 %.This apparatus is now being used to provide accurate measurement services to industry and is ready for inter-comparison and then developing transfer standards and reference material for checking experimental techniques and procedures.