old cast iron bath takes 2 days to drain away?

we have an old cast iron bath that takes 2 daysto drain away the u-bend has a very tight turn on it and I fear that it is "furred up" on the inside no amount of drain clearing fluids make any difference, thethin flexy drain clearingstrip cant get past the turn of the u-pipe. To make matters worse the cast iron u-pipe is then fitted onto a lead pipe that drains it away to the tank the lead pipe is running easy enough as the sink is connected into this and has no problem draining away. what is the best way to deal with this as I am not sure about how to seal lead back onto cast iron. we live in the highlands and have very peaty (acid) water