Cracks in that area are a fairly common problem on Commando exhaust pipes, especially the 850 systems with the balance pipe (you said "manifold" so I would guess you have the balanced cross-pipe system?) any slight misalignment (because the pipes may not have been manufactured exactly to the correct shape?) between the pipe and head joint or cross pipe joints can put stress on the pipes when everything is tightened up, which can cause them to fracture.
I know of other Commando owners that have bought pipes that are not quite the correct shape and do not always fit properly, and these tend to be the cheaper ones, but I doubt that even all original pipes were perfect?
If you do have the balance pipe system fitted you could change to the non-balanced type as that doesn't have the stress problems caused by possible cross pipe misalignment so it can last longer and is a popular mod. done by Commando owners as some say the non-balanced system looks better anyway, and two single pipes are normally cheaper.
The 850 MkIII has a different pipe to head joint arrangement which allows for a certain amount of misalignment, this arrangement can be used on the earlier models but MkIII pipes, seats and collets are needed.
Other things to check would be that the silencers are mounted correctly (on the rubber mounts?) As the whole exhaust system needs to move along with the Isolastic mounted engine. And that the exhaust rings are not allowed to become loose.
You could continue to have the pipes welded, but if they are fairly old then they are likely to split again? Or you could replace them, even better quality ones are not that expensive to buy? There really isn't any other option that I can see.
The Isolastics should not be set over tight, ensure you have at least the minimum recommended play, or you risk breaking the frame!
