We used to use those big heavy plastic things that are used to hold temporary fences up - construction work at an airport can be useful sometimes !!
bmi and bmi regional make use of these things as well, what look like the same "ballast blocks" weigh 20 kgs for mainline and 22kgs for regional. The only difference must be the heavier spray paint highlighting either "bmi lhr" or "bmi reg" !
