Andy Wade wrote:
The Natural Philosopher wrote:

If u can get the wood dry, and then apply a very runny resin to it
that soaks into the rotten fibres and stabilises (and seals against
more water ingress)

That's an essential preparatory step, IMO, and proprietary 2-part 'wood
hardener' products are available for the purpose. Any decorators'
merchant will have some.

Fibreglass resin is cheaper, and essentially the same thing

As to fillers I've been impressed recently with Polycell's 'Wood Repair
Pollyfilla' product[*]. It's a 2-part epoxy formulation, sandable but
retains some flexibility when cured. Car body fillers tend to set too
hard and may crack away from the wood on large repairs, IME.

IME sometimes the surrounding wood dries and shrinks away from the
filler after a few months. This is usually a one off event.

[*]
http://www.polycell.co.uk/products/p...polyfilla.html
(quite expensive, but worth it).