'It's gorgeous with floral decorations and elaborate designs'........uhmmmm, undeniably, and when you are fed up with looking at it please forward it to me.

The history of this pattern/design came, apparently, from the late C19 and was offered originally by McKee & Bros. as their pattern No. 140 - although after a somewhat chequered history arrived with Jefferson Glass Company in about 1915 as 'Star Line Krys-Tol' (but is apparently now more commonly known by the name of 'Diamond with Peg' - given, so the book says, by Arthur Peterson). Quite a practicable name really, when you look at the outline of the design. Your 'butter w/cover' appears to have been produced in 'Ruby Stained' and 'Custard' - and the thinking is that Jefferson was the only maker of this pattern in 'Custard'.
Most of the 'Custard' pieces seem to be decorated with roses, probably done by Oriental Glass, Pittsburgh, and according to the book both 'Custard' and 'Ruby'Stained' are very often found decorated as 'souvenirs'.
And now for the the value................and you must understand that I am quoting the value given in the Jean Loomis book, and as the author herself said........."a myriad of variables influence a price. At best, the numbers are only estimates, based the experience of buying and researching 'Krys-tol' for many years".........with the current recession Colin, values may have changed dramatically, and you would do well to contact pressed glass collectors in the States who may have a much more accurate idea of current worth.
Butter with Cover.............'Diamond with Peg a.k.a. Star Line.........Catalogue No. T 1335 for Ruby Stained - US$175. for Custard - US$275.
However, whichever way you look at it, very desireable.
P.S. Of course, if I am wrong on any of the above, I do hope the States collectors will put me right very quickly:)