An auction house might be a good shout, but a specific pottery type sale rather than one of those general type house clearance auctions. The problem with e-bay is posting the stuff. You could list it and insist on buyer collects.
A decent auction house should give you some advice, but names of individuals tends to drive prices more than the pottery company, think Clarence Cliff or Suzy Cooper for example.
Crown Derby is a big name, but it all depends on what your Mother has. Condition is also vital, anything damaged is worthless unless it is an extremely rare piece.