Fitness-for-service assessments of components operating at high temperature often require estimates of creep crack growth in service. Although methods for calculating creep crack growth are given in R5 and other codes, these methods are limited in terms of their range of application and can be over-conservative. This paper extends the current methods in a number of areas. First, a general expression is derived for relaxation of a reference stress for combined primary and secondary loading and significant in-service crack growth. Second, the reference stress history is used to develop an estimate of the transient creep crack tip parameter, C(t), for elastic-plastic-creep material behaviour for combined loading. This both extends current methods to more complex loading and reduces some known conservatism in these current methods. The general expression for C(t) then enables creep crack growth in service to be estimated using data collected from standard tests under mechanical loading. Finally, the paper presents some finite element calculations for defects in cylinders with welding residual stresses as validation for the new approaches.