Gerry Smith wrote:

In the house I'm renovating, some genius has plumbed for a washing
machine so that the waste water has to travel about 25 feet, round
three bends, to reach the drains.

As we live in a hard water area, the waste pipe has clogged up with
lime-scale, which is difficult (read "impossible") to reach manually.

Be grateful for some help: in particular, is it poss to use any
chemical to clear the limescale which won't eat through the plastic
waste pipes?

Taking lots of care, you can use hydrochloric acid as sold by swimming
pool suppliers. Dilute from the supplied concentration to between 5 &
10%.
Allow for the level to foam up and have a bucket of water handy to dilute
any spills and overflows.

None of the common acids you will be able to buy will eat through PVC
pipes