this ones slightly baffling me.......other than its still air circulating in system and its air locking intermitantly.what is worrying tho is why theres know pump over run wired into the boiler,its a big 80e which req.s it as per manufacturers.if its -over heating after 18 odd minutes the scenario which could happen is room stat satisfys,turns pump off and the boiler cant bring pump to circulate excess heat in h-echanger,thus o-heating.as its one u really need to be there i would get the plumber back personally