The word wankle is a derivative of the word 'wank'.
1) It can sometimes simply be used to mean wank (masturbate)
2) However, wankle can also imply that the wank is a smaller or faster affair than usual - this can be due to size of the genetalia (particularly if a penis is involved), or the length of time that the wank lasts (regardless of the genetalia involved)
1) Sarah: *knocks on bathroom door* 'oy, Sam, can I come in and clean my teeth, or are you having a wankle in there?'
2) I've spent all day checking out cheap porn on the Internet, I'm off for a wankle before the kettle boils