I think thats what Pomp was suggesting, to get them to have a look when she gets the MOT done. But imho, its much better taking a car for a pre-MOT anyway, the earlier ye can find out if it has any problems the better.
If ye take it to a garage, many will have a look for symptoms of problems etc for free and probably be able to give you much better idea's as to what it is.
Id swear it sounds like the Lambda sensor though, its the link between it mainly having the problem when idling, the car shuddering and the fact it getting worse over time (Lambda's arent detected straight away by the engine management light, and go gradually over a period of months usually), just sounds far too much alike the Lambda. But of course, there could be something shes forgotten to mention, or youve mis-understood Pomp.