UPDATE
At the weekend I replaced both eggsause marigolds and gaskets top and bottom (eggsauce to marigold and marigold to head) as well as the airbox and the MAF...
while I was at it I found that three of the plug leads had come off the plugs.. so no wonder it was running worse and worse..
anyway, the new maf and the good manifolds seem to have done the trick except:
twice I have had to stop and refit plug leads at the plug end that have come off.. each time it has been one of the ones that were off at the weekend: 4, 5 and 6. seems odd they keep popping off.. anyone know what can cause it?
on tickover the revs are very low.. so low that a couple of times it has stalled and when not stalling it sounds very rough and is shaking the engine in its mounts.. perfectly fine driving in fact quieter and more torquey than any time since I've owned it.. i can't seem to find any suggestion of a tickover adjuster on it... anyone know what the solution is?