I have a 33mm push on 90 Deg joint from my hand basin under the floor boards, which recently came off and flooded the kitchen. Am I OK to replace the joint with a weld joint using the existing waste pipe or would I need to replace the pipes too?

Thanks.

You would need to clean and prime both the pipe and the elbow interior and then you can weld with the appropriate glue. ABC and PVC both can be welded with Oatey All Purpose cement Regular Cements and Primers :: Oatey.com