Hi, and a bleated thanks for the replies. I had forgotten all about this thread until I got notification of todays post. I don't normally abandon questions like that, unfortunately my mum died unexpectedly shortly after.
Anyway, to follow up on the suggestions.
I do already run my own mail servers, but with one of my ADSL links down was cut off from my usual route out so redirected as a backup through Sky. When I found that replies had gone somewhere I wasn't expecting (I don't use or check my Sky mail account really) I started to investigate further. All the time my main (work) connection is up there isn't a problem.
I'll look further into the Sky settings, as suggested, and do some tests now, although there is a good chance I won't be sticking with Sky after their pricing changes recently.
Thanks again.
/aptanet/