Long story. Essentially I'm looking to get out of a lengthy marriage but over the last two years I think my STBXH has more grounds for unreasonable behaviour than me because I've switched off and shutdown thanks to long term unreasonable behaviour from him. I'm not sure how I progress with that?
It doesn't matter: the divorce petition is simply a means to an end. If you don't want to wait for two or more years to bring divorce proceedings, your only options are to issue proveedings on the basis of his unreasonable behaviour or (if there's a third party) adultery. If you thought it would get things off on a better footing, you could alternatively invite him to petition you on the basis of your unreasonable behaviour.