A thought occurred, that noise you can hear when you let off of the accelerator could be a vacuum leak which would be at it's highest when coming down from revs. You may have bumped one of those little hoses with your spanner when doing the coolant bleed. Being old hoses, one may have split, particularly where pushed onto a fitting. Wish I could say, just rip them all off and block them up, but because there are so many sensors etc connected to them doing god knows what, that's not possible.