I have used Ronseal wet rot wood hardener on my (house) roof where the soffit fits and it made a good job. 3 years ago and still good. After the hardener i used the filler where chunks were missing but if the wood is all there you wont need it.
I think you are thinking of Sean Barratt at www.tank.me I got my tanks from him and he made a great job. There can be a lead time for him to make them though.
Kev