if theres a hole in it all the metal around it will be weak and it becomes very time consuming to try and fill in all the new holes that the welds create i did it not long ago on my old pipes and i spent more time filling holes than i did removing all the studs i snapped taking them off.... bought a new set of stainless ones
if theres a hole in it there will be more shortly get on ebay and get a stainless steel set
Try some chemical metal or some of that 2 pack epoxy puty that comes in a tube, it should be better then gungum.
Should be ok for a short term fix to get you through the MOT but it will probably fall out after a few heat cycles.
Make sure the downpipes have no oil or carbon on them or it probably won't stick.
Did not think of chemical metal...what a fool!!! Thanks for that..have some in the shed left over from repairing the engine cover on a CB 500. Also used it to repair small holes in my Discovery and they have been fine for 2 years!!..
Will clean up the area tomorrow since I have all the bodywork off just now...looking for an oil leak I did not have!!..d'oh!