Call NHBC in the first instance. They will send out a 'risk assessor' or similar to assess whether their is a claim. (This assumes that your property was built with an NHBC warranty.)
Its an odd place to have water ingress. But if you have rainwater coming in over a lintel, that suggests that their hasn't been a proper cavity tray installed over the lintel with weepholes to allow water running down the cavity to drain outwards.
It would be worth checking that there are no issues with the guttering that are allowing water to penetrate the cavity from above.
Once you have your report from NHBC, you should note that they are an insurer and as such will be looking for reasons not to pay out. It is likely that their report will come back claiming there is no fault with the property. At this point, you will have to pay for a Surveyor to write a professional report. Assuming this confirms there is a construction defect then NHBC would pay this money back as part of the repair.
Its also worth asking the neighbours if they have had similar issues in the past.