I think the shift was caused by the 3 bolts becoming loose...they didnt feel very tight...
Driving around today I could hear a hissing when I put my foot down, took it home and leak tested - no problems.
Later found out that the pipe that goes from the exhaust manifold to the EGR valve (which has been blocked off) is leaking on the bendy section, to make matters worse the nuts holding it onto the manifold are rusted beyond recognition, I think for now I'll pad it with exhaust paste and wrap it up in something, then jubilee clip it to hold together...not ideal :/