Philippa: Aber is indeed a Welsh word, meaning the mouth of a river. However, my gaelic-speaking project manager here informs me that it's a bona-fide Scots word for the confluence of two rivers.
Incidentally, the Welsh word for the English is 'Saeson' (quite a lot like the Northern equivalents), but the word for England is 'Lloegr', which is what we called it when we ran it.