i have used the big grip ladder stabiliser on ebay. It works well, its annoying carrying it around, but works well, i have had my ladder at around a 25 degree angle with this and up pretty high and no slipping. I hate jobs where the ground is uneven, this is handy but not allways great, because when you adjust the sideways angle its not allways where you want it, you can adjust the feet but it gets quite tedious. I have seen a half circle adaption for sale that bolts to the end of the ladder and can rotate to any sideways angle you need on its two feet. Local tool shop.