What you need to do (if not yourself, then by someone who 'knows') is dig around enough to see if you find clean dry wood within a reasonable distance of the corners, then cut out the bad and replace with good wood. However, you also should trace where the water's coming from because if you don't deal with that, you'll just be back where you started very quickly.

If the repairs are NOT structural you can use Minwax High Performance Wood Filler to restore the surface.
The hardener is also useful after you remove the rotted wood to seal the surface and harden it up to accept the patch.

There is an impregnating epoxy product called Clear Penetrating Epoxy Sealer [you can easily find it with a web search by that name] that actually selectively wets the parts of wood that fungi ate into. It finds all the rot, because it follows selectively those areas where rot fungi ate along the grain. It is a little-known scientific fact that rot goes into wood farther than you can see. The manufacturer found a way to make this visible. The science of it, with time-lapse photographs and pictures taken through microscopes, is on the web at www.woodrestoration.com.

All woodrestoration has shown is that low viscosity liquids penetrate rotted wood.

Not that other products do not, just that there product does.

Rotted wood is porous.
Surface tension will pull low viscosity liquids into the pores.

Ever wonder were the first application of masonry sealer goes?
Demonstrating the obvious is far from a breakthrough.

Like Lucy says tear out all the bad wood and replace it. This epoxy and other stuff may work for a while but if the leaking problem is not fixed it will come back and be even more fun (expensive) to fix. Make sure the flashing is installed correctly. Start at the bottom and work up overlapping as you go.

Of course water intrusion must be fixed. Flashing and caulking, properly done, are always mandatory.

As for the website woodrestoration.com, it helps to read the whole thing and understand it. In Section Four there are actual mechanical tests that show a considerable increase in the strength of treated wood, but the really interesting thing is that the treated wood is just as flexible as natural wood.

So, the remaining wood fibers are glued back together, treated wood can still breathe and it behaves close-to-natural. Sounds like restoration to me.

Thank you for taking the time to answer my question. It's not a project I'm qualified to fix so will find a carpenter to take a look see.