Rotten wood in house wall

Hi all,
I have removed the plaster and concrete render from a wall under the stairs to find the source of a damp patch that was showing thru.
I have uncovered what I believe to be the source which is a long piece of wood built into the brickwork of the house. The wood is damp and starting to rot, (see pics).
I am not sure why the wood is there? It certainly looks newer than the house, (which is around 100 years old).

Can anyone help with the best action to take? I need to treat the wood for rot and prevent it from causing damp in the future before I re-render and plaster. Ideally I don't want to have to attempt to cut the wood out. I have previously repaired the external render on the house so I believe I have solved the original cause of the dampness.

Any help would be gratefully appreciated.

Timber was sometimes put into the wall by the bricklayers to level the courses. Back then there wasn't a standard brick size and the interior commons were often a different depth to the face bricks. A length of timber would level it up.
Many of these timbers are now rotten and can cause damage to the structure of the wall. If they are really bad its best to cut out a bit at a time and replace with brickwork.

Thanks a lot for your response. I was afraid you might say that. The problem is the wood extends into the sloping cieling under the stairs and behind a door frame so the work involved is huge. I wil bear it in mind.

Thanks again.

If you have cured the ingress of water, u could just treat the timber for wet rot and leave it to air for a few months.

jac - stuart is spot-on with his explanation regarding levelling-plates. In addition they could also be used, when adjacent to doorframes & windows, to allow these elements to be anchored via cut clasp nails (Oh how the Victorians loved their old nails for holding everything together!!).

The fix is straightforward, either treat and leave (as per Poo's advice) or chop out the offending section by drilling & chiselling, treat the stump that's left, fill with brick/sand&cement mortar.

An observation ... timber moves (expands/contracts), masonary doesn't; these old walls would have been originally plastered with a lime plaster which tolerated this slight movement. Gypsum plaster will tend to hair-line crack along the line of the levelling plate (nothing serious) and may show through any decorations.

Thanks a million to you all.

It's good to know there are such well informed people out there to help.

I have noticed that the dampness in the wood is drying already, (1 week), and the only rotten section is at one end, so I will leave a bit longer, treat with wood hardener or similar and render over as suggested.

It has been suggested to me that I should put a piece of plaster board over the wood before rendering as a barrier. Would this seem a good idea to you or should I consider an alternative or render?

Thanks again to you all.

jac - we tend to use expanded metal mesh (it's galvanised) over these plates. You can buy it in rolls (wider than the timber you want to span) then nail (with galvanised clout nails ... big flat heads or plasterboard nails) the mesh to the masonary and the timber. This mesh forms a key for the render (the render is unlikely to form a long-term bond to the timber and will crack when the timber 'moves') so the mesh fixes this problem.