well I'm sitting here feeling rather dazed (and relieved) at how things went.
I started the conversation by saying I wanted to have a positive discussion that resulted in a workable compromise about DSS having a key. DH said straight away the it wasn't ideal to give him a key, but as we've given him a real push towards independence by buying him a car and driving lessons, that he should now have a key "providing there is some control" (DH's exact words).
So we've agreed on the following:
If DSS wants to visit, he rings/texts one of us first, so that if we're not going to be in, or it's not convenient, then we can say so.
Neither of us mind if DSS wants to come over straight from sixth form college in the evening, if he rings first, and lets himself in, as I'm usually back around 5.30pm, so he won't be alone long.
If he wants to stay in the house for a short while after we've gone out, we'll arrange a check-list to help him lock up.
So all of the above negates my fear of him coming over to an empty house without our prior knowledge, and he shouldn't be in a position where he spends lengthy periods unsupervised, or is alone long enough to try to cook/use kitchen appliances.
And the best news of all, is that DH was in total agreement that if we're going away on holiday, or for the weekend, that DH and I secure the house, and that DSS doesn't come into the house while we're away, as there's absolutely no need.
All this was sorted out in approx 5 mins, no raised voices, no lost tempers. So basically if DSS wants to arrive shortly before us, or leave shortly after, then he can follow a check list and then use his key. It seems that DH and I were on the same page after all. It gives DSS a little responsibility whilst we remain in control of our lives and home. Can't argue with that.
Thank you to everyone who posted support/opinions on my earlier thread.