The ageing of a 20/25 Nb stainless steel AGR fuel cladding alloy at 1023 or 1123 K is shown to produce small but significant changes in constant stress uniaxial creep behaviour in tests at 1023 K. The present paper is addressed to a correlation of the observed creep behaviour with the microstructural developments caused by the ageing treatments and the subsequent thermal exposure to 1023 K during creep testing. In the present alloy, NbC is unusually unstable with respect to G-phase (Ni16Nb6Si7), with the latter forming more rapidly and at higher temperature than in other comparable materials in the absence of irradiation. Consideration is given to the reasons for this. It is shown that the G-phase particles, in contrast to those of NbC, are deformable at the 1023 K creep testing temperature. The creep behaviour is interpreted in terms of this observation, together with those of the progress of the NbC to G-phase transformation with ageing/creep, and the particle distributions thereby produced.[/p]