They did. Then DaFT told them they couldn't have them all, put pans on them and sent them over to the WCML sharing them between Silverlink and CT (both now LM) where they are known as 350/1s
They also put out a far more recent tender, in 2009, for 120-180 vehicles compatible with their existing stock, that most commentators expected to be either complete 450 based units, and/or additional '5th cars', based on this:
120 to 180 vehicles (20 metre) to operate in 5 car formation. (or approx. 100 m length equivalent).
The result of that tender turned out to be the 458/460 merger, and the 456 transfer; given that provides only 108 vehicles we can see why according to the rail press they still want more units...
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Bigger fleet of 444s instead? Shame the design is no longer available.
A similar 23m end doored unit to a 444, probably based on the 380 bodyshell, would be available if they were needed. Siemens would build whatever they were asked for.
Please don't repeat the rubbish that a reputable manufacturer doesn't keep the drawings for an existing product, and can't make compatible add on quantities of stock only ten years on, even if they have to conform to revised crash worthiness standards...