The catalog says, "800 Hi-Line Corner Base Unit." H890 x W1031 XD631. The width 1031mm doesn't not reflect to the name of the model. Looks like the first three Corner Base Units model name does not match the Width. The pdf that Howden provided Tewkesbury "Range Summary" seems to be missed typed saying 800. If you look at all Sink Base Units for example 800 Drawer-Line Sink Base Unit (LH & RH) H890 x W800 x D575. The width 800mm does match the title/model in the pdf. See images please.
"Where do those 1031 and 631 numbers come from?"
They came from the Howden catalog see image. 800 Hi-Line Corner Base Unit - Height 890 x Width 1031 x Depth 631.
"I can choose between 3 models, "800", "900", "1000". What do those model nos refer to? Something in the Howdens naming system? Don't think so. The basic width? Well, an increase in 100 in the model number seems to be reflected in an increase in 50mm in the actual dimensions. So, doesn't look like it."
Please see Howden catalog PDF. H890 x W1031 x D631 and the dialog box in VW does have it correct. The Model #800, 900, and 1000 are wrong for the first three Corner Base Units. Need to be fixed in the VW dialog box to say Model - 1031 Hi-Line Corner Base Unit to match Height 890mm Width 1031mm and Depth 631mm and etc.... Look at SInk Base Units and these match the Howden PDF catalog see image.
-Overall, this will be fixed in future release.
Thanks.