Hi Sue(M),
I'm not sure what you mean by undesirable features in themed weights. The Christmas weights from Whitefriars were all based on Christmas topics (some religious, some not). The last two, of the Partridge in a Pear Tree and the Christmas Bell, were viewed as excellent technical examples and desirable in their own right as well as being scarce. This boosts their market value as does the fact that many collectors would buy the others and then want to have the complete set, thereby increasing the scramble for the scarcer ones.
Cheers,
The christmas bell is not viewed as an excellent example. The tale is that the 1980 was supposed to be shepherds and sheep, and there are sheep canes in existence, but it was not successfully prototyped and in the end, the bell was done as a simple cane setting so that a 1980 xmas weight could be produced. Of course 1980 was when the factory closed and the number issued was low. This means that the 1980 is the most expensive of the xmas weights because it is rare and there are not enough to complete all the sets that collectors (like me) have.
Note also, over the last couple of weeks there have been a number of very complex garland and star settings on ebay. The stars particularly went for enormous sums (£500). Again, this is because of rarity.
That is really the determinant of the values of the weights. Not theme/design, but how rare/unusual they are and so how many collectors are missing them from their collections.