Thanks to both of you for your responses. I have been studying the material from the various links, much of which I have read before. I could still use more study. The problem at hand is very sensitive to the start point, so I have been looking for methods which would seek a local optimum, rather than get distracted by a global search.
I am finding it difficult to discover what methods are available to NonlinearModelFit. Many I try, including methods from FindMinimum and NMinimize return either as not available. Returning:
"Value of option Method -> \!(\"FindMinimum\") is not Automatic, \
\"Gradient\", \"ConjugateGradient\", \"InteriorPoint\", \"QuasiNewton\
\", \"Newton\", \"NMinimize\", or \"LevenbergMarquardt\""
Or not available for constrained problems:
NonlinearModelFit::ucmtd: Method -> Gradient can only be used for unconstrained problems. >>
In fact, the only method I find accepted (other than automatic) is Interior Point.
I will of course continue to study, but if anyone knows of documenation on methods specific to either NonlinearModelFit or FindFit I would be grateful.
Thanks and kind regards,
David