Thanks for the comments Chis.

I am thinking that I will try to slip a 3" pipe over the 1 1/2" and put an adaptor on the end for the garden hose and then try to force it down and along the old 1 1/2 inch, flushing the sand as it goes. If it encounters any clay or rocks I'll be
SOL and have to invoke plan B which is what you suggested.

I think this must be a pretty common problem with people that get undersized systems to start with. One would think that someone out there has a solution.
Gee if they can tunnel under the English channel to France ......