Given past experience, especially when called in to sort out heating problems after a new build or renovation, I would recommend you also get an experienced plumber to 'snag' your plumbing cos snaggers never seem to give it a second look!
sorry I have no idea how much someone would charge, would rather get someone trained on the energy rating, than an actual plumber to do it, maybe even talk to someone in this regard before getting any heating work done, as they will have something to say for you on zoning & control of heat etc which will save you buckets as the years tick by