As there have been over 1100 visits to this thread I feel I ought to bring things up to date.  After the three weeks or so of having my outgoing emails blocked, suddenly everything returned to normality as reported above and I happily sent emails as normal.   
We have builders in, and some weeks later they switched off everything for over 24 hours.  On resumption of Internet connectivity we found we had been issued a fresh IP address (as one would expect) but when we tried to send an email found we were unable to, as we had been (Oh! You guessed….) blocked.  Again I checked all the internal IP addresses and scanned everything to the deepest available depths without any viruses being detected,  and then opened up an old laptop that hadn’t been used since February this year.  And what a surprise?  Outgoing emails were blocked – proving beyond any doubt my equipment was NOT at fault.  And yes, I did check that routers and modems do not have the same sort of IP address as the emails and so cannot be responsible.  I spoke with Nigel Morton and the Eurona help desk and sometime later normal services were resumed without any feedback, apology or reasons given.
All is well?  Not quite.  Over the last four weeks we have found that on numerous  occasions the Eurona telephone line was dead, even though the Internet worked perfectly.  The only way we know the phone was dead was when someone called on the mobile and said the obvious, or we noticed we had only two lights on the Eurona modem instead of three.  Each time I texted Nigel Morton, and sometime later the line was restored (without feedback so I don’t know if he did in fact do anything).
However, the last time, Saturday, I received the following text from him, “I can’t do any more than advise (you to) reset the equipment as there is a change in the support procedure.  For more support u need to phone Eurona 910444888” (My emphasis) So – if you thought you have local help from your local agent think again.  You will have to phone the help desk and hang on, and hang on - at mobile charges of course as your landline will be dead - Nigel Morton did not elaborate on what the changes are.   
Am I being neurotic in thinking I’m getting a bad deal from these people and why am I  losing an important connectivity?   Unhappily Eurona offers a faster speed at a lower price than Iberbanda….but one has to wonder which gives the better service over all.  And now – dare I turn off the modem for 24 hours and get a new IP address?