Thank you for your reply, but I have now got to the bottom of things and my game is definitely lost - my husband overwrote my local save whilst on my PC in his Origin log-in!
Origin are now looking into how this could even be possible, because on his log-in he shouldn't have been able to have any effect on any saved games that were saved under my log-in details.
But he did have an effect - he overwrote a 340 hour game that was saved on my PC.
My game wasn't saved to the cloud, it was only saved as a local save on my PC. When he tried to come back out of his game on my PC he was asked what he wanted to do about the local saves that had been found. He didn't understand the technicalities of what he was being asked (he's not what you would call a techno-buff - he's not been gaming long), so he just chose one of the options, and he isn't even sure which one he chose or why he decided to go with one option over the other, but he must've chosen an option that basically meant 'replace'.
This, I suspect, is something that happens a lot in the computing world i.e./e.g. Joe/Jane Bloggs is asked a technical computing question or given technical computing facts, which he/she doesn't really understand. He/she is then given two action options related to the question that they didn't really understand. The option chosen by Joe/Jane is surely then no more than a stab-in-the-dark! My husband didn't understand the information given and, therefore, didn't know which option was the correct option for the situation and just took said stab-in-the-dark!
So, the outcome is that I've lost my game, and now when I log into my Origin I have access to a copy of his game, which is the only DA:I game on my account, and I can play it, and I can even play at the same time as he plays a copy of his game on his Origin log-in on his PC, sitting next to me! But my game doesn't exist anymore, not on my PC and not on the cloud.
Origin agree that he shouldn't have been able to even access anything saved under my log-in details and are now looking into how this could even have been possible.
What I would suggest to anyone who plays through Origin is that they ensure that the 'Enable cloud storage for all games' box is ticked before you start off playing through an Origin account - for those who aren't sure how to do this you can access this by right-clicking on the picture of the game concerned in your Origin account and choosing 'View Game Details'. This takes you to a page with two tabs - one labelled 'Details' and one labelled 'Cloud Storage'. Click on the 'Cloud Storage' tab and make sure that there is a tick in the 'Enable cloud storage for all games' box.
There is every chance that if my game had been saved to the cloud then my husband MAY not have been able to overwrite my game, but it doesn't change the fact that he did this under his log-in details and that my game, saved under my log-in details, wasn't securely non-accessible to him as I feel it should have been. This issue is now in the hands of Origin and hopefully will, further down the line, help to ensure that this can't happen on anybody else's account - it's a problem that needs fixing by Origin.
Hope this post helps others to safeguard their games by ensuring that they know how to save to the cloud and can choose to do so, ensuring that they have an external backup of their game that is safe from PC issues at home.
I now have to go try to gather together the motivation and heart to go begin a new DA:I game from the beginning - what a p****r!