from your description I am guessing that you are in the UK. If so and you have a standard brick/cavity/block wall they used to usually have a piece of 18 inch DPC put over the top of the arch to prevent water runnig down the inside of the external skin and inside from the top of the arch. Arches have always been a problem in cavity walls in the past.
However nowadays your builder should have inserted a purpose built cavity tray as shown below. The fact that it now leaks shows that it has been incorrectly rebuilt. Putting a sealer over the arch is just a bodge.