Water – Toilet leaking – is it the washer

Are you sure it's actually leaking from the bottom seal and not just flowing down from the top seal connected to the cistern? To be sure run your dry hand around the pipe near the top seal.

In any case to replace any of those seals:

1. Turn off the inlet tap to the cistern and flush until no more water remains in the cistern. (Have some rags ready anyway as there's bound to be a small amount of water coming out residual to the cistern.)
2. Undo the nut at the cistern end by hand and lower this end of the pipe--now you can slide it back towards the cistern to remove it from the bowl.
3. Replace any broken seals and reinstall (reverse of removal instructions).
4. Open the water to cistern inlet and flush several times to check for leaks. Make sure both the cistern and bowl are well secured as any possible movement left may cause leak during usage.

That looks like standard UK 15mm copper pipe with an olive and compression fitting.

The correct term for the "washer" is an "olive". It is made from copper. If it is damaged, you will have to

• buy new "15mm compression olives", they often come in a pack of 5.
• cut off the old olive (you can get olive splitters to do this job)
• clean up the end of the pipe with wire wool (or similar) until bright and shiny.
• make sure the end is circular and undistorted for at least 1 cm
• make sure there is no burr or lip at the very end (use fine grit sandpaper)
• slide a new olive over the pipe (above the compression nut)
• ensure the parts to be connected are clean
• screw everything together

So long as everything is clean, smooth and undistorted, you don't need any plumber's gunk, gloop or PTFE tape.

Don't overtighten, if it weeps you can tighten a 16th of a turn or so.

If necessary, you can cut the pipe further down and fit an extra joint with a short length of new pipe, you might be able to use a flexible connection hose for future convenience - some of these can incorporate an isolation valve. A professional plumber would use an end-feed soldered copper joint but you can get ring-solder joints that just need heating with a blow-torch. Compression joints are easy and compact (and my preference) but you can also use push-fit connectors (HEP2O, Speedfit, etc) so long as they say they are OK for copper as well as plastic pipe.