On Sat, 23 Jun 2007 14:09:40 GMT, Geo
wrote:

Daughter has a couple of garden benches and one has a broken rotted seat slat.

Think carefully about replacing the lot. If one is rotten, chances are
the others are too.

Difficult (for me) to tell the wood under all the varnish but probably oak.

It won't be oak. Nor is oak, in anything under 2" thick, a good choice
for outdoor work.

I'd go for a resinous softwood like larch and I'd select my board
carefully to find the most resinous. If I insisted on hardwood, then I'd
use chestnut as that's pretty rot-resistant. Oak isn't a good choice
here. Larch is also one of the cheapest timbers you'll find.

As a finish I'd use either Danish oil (Screwfix, cheap) or one of the
Organoil finishes (harder to find) as they're quite impressive for
outdoor woodwork. Then I'd leave the rest of the bottle on-site and
tell them to re-oil it annually.

Any suggestions for (preferably local - Central Scotland) source

You can probably find it locally grown.