Smoking Joe,
Never found anyone willing or able to take on cast iron gutter but got silicone reapplied. So far it seems to be a better job than before though it's yet to be fully tested. Old dog's open cell foam sounds interesting- might do a better job. (In our case the line of the gutter from next door is out of kilter and to really fully repair it might be a big job spanning several houses in a terrace- so compromise solutions may have to do...).