Mine was doing exactly the same thing, Got the soapy water thing done and lo and behold a porous airbag. My Disco is a '99 model and had done 280,000km before it played up. I ended up replacing the airbag with a Dunlop one from Airbag man (he was the cheapest one I could find, and no I don't have any affiliation with him) and after receiving it 10mins later it was all fixed Oh and $385.00 as well.
Hope this helps
Doc

