I've fitted a new weather-bar at the base of an external door, bedded on plenty of silicon-based caulk for good measure. Water still seeps into the house at an alarming rate when the wind is up. My suspicion is that the water ingress is through the "sandy" concrete base on which the weather-bar is installed. Is this at all likely - and if so what might be the solution please?

It is very possible that water is coming up through concrete.

U can use a water sealer paint on outside concrete. You will have to go down below the slab height.

If water is coming up from under the slab, check to see what kind of water sealer to paint the area with. The problem you may have is that the water may move into the house more for it will seek the easiest route out.

I have used water proofing paint with every good out come.

Check with your big box store. Be sure to read the directions and that it can be used for what you want.

Chuck

Thanks Chuck - I'll give what you suggest a try.
Roger