Andrew Gabriel wrote:
One wall of my dining room is pine. I like the colour of newly
sanded light pine, but this has darkened since it was last sanded
and varnished (probably 20 years ago by previous owners). Before
I embark on the rather mammoth and incredibly boring task of sanding
and varnishing it again (previous owners said they would never have
started it if they had any idea how much work was involved), is
there something I can do to prevent it darkening in the future,
such as a special varnish or some pre-varnish dressing?

1) Paint it.
2) Paint it - "graining".

HTH.