Hey Slimjim, how do you get inside the water jacket? unless you cut the outer jacket, it is impossible to get between two wall of steel to do any welding. I've been welding for 40 years and don't know how you can get to the inside jacket, it's just to narrow to squeeze into a pipe and do any welding in a 2-3" area.
The best way I know is to empty the water and find out just how bad the back of the boiler is. It could be that most of the jacket is gone or just one spot is bad, but I don't think so. I've ran into this before and if there is a small hole, chances are most or a good part of the inside is almost gone. If you only have one bad spot, cut out the hole and make a patch 1 in. larger than the hole. Weld with a 7018 rod. vertical welds should be welded vertical up. I been a certified welder since 1994 and you need to get a leek proof weld. This is the only way I would fix a leak.