Hi chooksandeggs-Aus, 
 
That is so fascinating!!  will the resulting sliver (Silver Legbars will be the correct name?) lay white eggs from the Leghorns?
		
		
	 
 
Egg Color 101.
Blue X Blue = Blue
Blue X White = Blue
Blue X Light Brown = Green
Blue X Dark Brown = Olive
 
White X White = White
White X Brown = Light Brown
White X Dark Brown = Brown
 
Since the Silver colored Cream Legbar had parents on both side with the blue egg gene it is a Blue X Blue and it will lay Blue eggs.  
 
I think the confussion is that the Silver Legbar breed Standards call for white egg and non-crested birds.  The silver color however is NOT linked to egg color in any way. The Silver and Gold varieties should probablly be treated as a different breed since they were created without the use of the blue egg laying hens form Chile. 
 
I would propose that what we real have is the following:
 
Breed 1 - The Legbar (White eggs & Non Crested)
1) Gold Variety - Standard of Perfection
2) Silver Variety - Standard of Perfection
 
Breed 2 - The Cream Legbar (Blue Eggs & Crested)
1) Cream Variety - Standard of Perfection
2) Gold Variety - Non-standard
3) Silver Variety - Non-standard
4) White Variety - Non-standard
 
I would suggest that the non-standard Cream Legbars keep the word "Crested" in the name to destinguish them from the white egg laying and non-crested variety (i.e. "Crested Silver Legbar" for the non-standard Cream Legbar, and "Silver Legbar" for the white egg variety per the UK standards).
 
The 
Gold Legbar breed was standardized with the Poultry Club of Great Britain in 1945. 
The 
Silver Legbar breed was standardized with the Poultry Club of Great Britain in 1951. 
The 
Cream Legbar breed (origianly know as the Crested Cream Legbar) was standardized with the Poultry Club of Great Britain in 1958.