Get a qualified plumber to do the work. They'll probably just drain the tanks but if appropriate they'll be able to put bungs in the tank outlets and/or freeze the pipes below the basin. Also discuss getting them to fit some isolators...
No problem! Buy a pack of dry ice, carefully pack the dry ice round the pipe. It will freeze in a few minutes, Carry out the repair. Take away the dry ice and warm the supply pipe a bit. Done!!
However. Buying a bit of dry ice was...