[prsns]

I have fitted a new bathroom suite and now when I pull out the plug in the bathroom sin the water flows back pushing the water in the bath trap into the bath. The pipe work has been in place for 20 years and there was no problem before.

[speedball1]

i have fitted a new bathroom suite and now when i pull out the plug in the bathroom sin the water flows back pushing the water in the bath trap into the bath. the pipe work has been in place for 20 years and ther was no problem before.

Well you have a problem now. You have a blockage downstream from the tub. The solution would be to snake the drain line from the tub waste and overflow, (see images) .
Unscrew and remove the overflow plate,(The one with the drain lever) and pull out the tub stopper. You now have a clear shot at the trap. Now feed the cable( 1/4" best or 3/3" with a small tip. You can rent a Ridgid K50 sink cable.) About 2' down you will run into the bend of the trap. Crank and push at the same time to work around it. Your blockage will be found in the lateral pipe going to the stack. You shouldn't have to put out more then 10 or 12 feet. I have found tub and shower clogs to be mostly hair and the worm should auger into it and pull it back. Hope this helps and thank you for rating my reply. TOM