Anyone (meaning everyone) needing more blue safety belt material, and not wanting to go grey waiting for a resupply from Hornby: it's merely a common type of hair ribbon called "Grosgrain". Find matching blue colors locally or online in 3mm and 5mm and have at it.
Looking at photos of the real belts the Grosgrain texture supplied looks too coarse for 1/8 scale, so maybe try some standard ribbon too.
http://scaleautoworks.com/images/Lotus72belts.jpg
It seems the kits supplied enough material according to the lengths called on page 30, without acknowledging you need "x 2" for each length.