Leaking bath from drainage pipe!! help!

[Jfrost]

My bath had a slight leak from the drainage pipe, where it joins the bath - I think. But yesterday it was a lot worse and took several hours to stop dripping from the ceiling of the room below. I think it would be best to replace the joining pipes from the plughole to the basin waste pipe just ensure all problems are fixed, as I do not want to cause more damage to the ceiling by gettin it wrong.
I have never installed a bath so and step by step tips and advice would be gratefully recieved on the best method and what not to do.
Thanks

[EyeTry]

Your leak could be from the metal waste outlet that protrudes through the bottom of the bath itself OR from the (usually plastic) bath trap that screws onto the end of that mw outlet (there will be an Inlet and an Outlet with that trap so two potential leak points - and if it is bottle-type fitting then from the base that unscrews for cleaning purposes) OR from the oultet joint and the pipework.

Stick an old roasting tray under the outlet area, put a pint or so of water in the bath then use some paper kitchen towel / loo paper wrapped around the mw outlet and see if that becomes wet. If No then move the paper to the bath trap inlet joint, then outlet joint. Hopefully one such test will reveal the problem site and by working top downwards you should identify it correctly.

It may be a nut on the mw outlet or the collar on the bath trap needs a bit of an extra turn (a spanner on the metal waste outle but use just hand pressure on the plastic bath trap and any plastic compression joints....)
However a full job shouldn`t take much more effort.... you could unscrew the collar of the bath trap to the mw outlet, move it away from the outlet then lift it a bit higher so as to move some water from the trap and away down the pipe OR just twist the trap on it`s side so that the water in that trap comes out into the tray.

Disconnect the trap from the pipe (undo any screw collar or twist any push-fit joints apart. Check the black rubber ring/collar inside the joint for any damage or it being pulled free from its recess. A drop of washing up liquid or silicon will lubricate a push fit joint prior to reassembly).
Long nose pliers inserted in mw outlet should hold it whilst you usea spanner to undo the locking nut on the under-bath side of that outlet. Check the state of the washers bath side and under side and replace if necessary. I`ve found LSX1 sealant (in a tube like toothpaste) from Screwfix, B&Q, Plumb Centre is excellent - run a bead of that around bath hole BEFORE you push the metal waste outle (MW) into position, then tighten the nut on the underside of bath. You could put some on the starter threads or just wind some PTFE tape around the threads before screwing the waste trap into place. Reconnect the pipe to the trap outlet.

Check all joints for security - but don`t be tempted to get white knuckle tight or you will either split or at least distort the joint!!!
That can all be done without moving the bath or are you wanting to move or change the bath anyway? Good luck, EyeTry.