Hes having a nightmare trying to fix it, he can’t get an engineer in who can fix it. If pump ain’t running at full speed there’s a restriction somewhere. Bleed the highest radiators. Take of valve head, strip diverter head. Check cold feed is not blocked. Pretty simple stuff.
Or buy a hive wired stat off eBay, no power required. https://www.ebay.co.uk/itm/303536177697?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=e3b4ndx4s6k&sssrc=4429486&ssuid=2w-QAnO7RlO&var=&widget_ver=artemis&media=COPY