Update for anyone interested in this stuff...
Bought 50 of these things for £16
http://www.ebay.co.uk/itm/291654257354?_trksid=p2060353.m2749.l2649&var=590688243181&ssPageName=STRK%3AMEBIDX%3AIT
They look like crap on the back side of 'em, but will be great for anywhere the back can't be seen. 10 of 'em to a pack, one pack is 2.4m so not bad value, should look ok, and take a few hits with the rally car. 12m in total in the pack and just bang into the ground, and clip together to form a continuous wall.
Other barriers I've been making today (off work with a goosed back) are as follows: red/white hazard tape across end of twin walled plastic sheet (like this https://www.roofingsuperstore.co.uk/product/corotherm-4mm-clear-twinwall-polycarbonate-roof-sheet-1220mm-x-610mm-x-4mm.html?gclid=CIKruemXt84CFSgz0wodzAAJVw) opposite to the direction of the box sections, cut along tape, put a few tent pegs down through the box sections... Nice movable barrier that can be bent around corners.