Maladministration is the misapplication of a body's own rules and can only be determined by the ombudsman in the case of certain bodies (incl. EH) and he will only hear complaints via an MP if the latter hasn't got anywhere. BUt, as I said, it's not a route that's worth persuing IMHO.
I agree Brigantes' advice is very good sense. I'd only add that the demand should also be for openness as well as action. Otherwise, they could simply say yes, we WILL repair it ASAP and our demand for it to be done would be answered. If they're open about the plans the public has a chance to comment and be reassured.