I was on my way to work when I heard a loud clattering and a slight loss of power. I pulled over to have a look under the bonnet, and noticed how loud the engine was when idling, it sounded like the exhaust or manifold was blowing. I could feel exhaust gas eminating from somewhere in the engine bay. It turns out one of the bolts fitted to the exhaust manifold had come undone and vacated the premises. Figuring I wouldn’t be doing too much damage, I continued to work and back home and replaced the bolt. Unfortunately the cream coloured rocker cover is now discoloured from exhaust gases, so I need to clean that up at some point.
Edit, it happened again! This time the other bolt. I got the garage to replace it and seem to have gained another 0.3bar boost. It seems exhaust gas must have been escaping slightly from the loose manifold bolts. Now the bolts are tight, it is boosting correctly and the fuelling is still fine under load. She feels verrry nippy now. Free power!