I would recommend a 'Ladder Mate' http://www.laddersalesdirect.co.uk/laddermate-p-301.htmlthere are cheaper ones.
I would recommend ladder twins I have used them on frozen deck to clean gutters and they are excellenthttp://www.laddertwins.com/buy.php
ive been doing house recently which has decking in back garden and even though my ladder has swivel feet fitted im a bit nervous while on it ! was racking my brain trying to think of a way to make ladder safer and ive been to my local hardware shop and bought a coulpe of door wedges . im going to wedge them in gap in decking and see how that works
if the decking is dry and the lines in the decking are not vertical from where you climb the ladders and are horizontal i use my rojak.IF THEY ARE WET OR VERTICAL LINES FROM WHERE I CLIMB THE LADDERS(OR BOTH!!)I USE AN EXTENSION POLE!!decking is very dangerous also if there is green moss on it.no way in a million years would i risk it!! there was a guy on here years ago that said put a scrim under the ladders and away you go!!MADNESS!!AWAY YOU GO FLAT ON YER BACKSIDE!!! : :