you could try putting in a couple of air bricks to help or however replace the plastic sheeting with felt although more expensive the felt will be a lot better protection from the weather
there could be mortar stuck on the ties in that patch of brickwork and it is leaving damp spots or a tray could be filled with mortar, however these should be checked on an inspection perhaps it could of been missed.
hi there if the lintel looks rotted or weak or shows obvious signs that its degraded, then change, however it mite be worth just changing it and putting in a standard concrete lintel anyway there cheap enough and will last longer, easy enough to do.
