STL are Intergas installers, they come and quoted for me last month but come back some silly amount so I told them I wasn't interested, been pestering me via text ever since, seems like a lucky escape for me.
In that case, I would recommend that the op contacts Intergas directly to tell them that their recommended suppliers are fitting massively oversized boilers and leaving the installation inoperable. I would then ask Intergas as a reputable company what they can do for me. Maybe they can lean on the installers a bit to correct everything, or may even help directly. Worth a try.