Hi Hewy,
That's great, I'm sure you'll find it a lovely setting.
With regards to the Asterias, we had around 29 people (about 8 kids) and the Asterias I'm pretty sure was closed, however I can't guarantee that this was the actual case. When myself and my wife had went for a meal there before we had actually decided on the Asterias (we chose the location after arriving). It was very quiet at the time and I maybe wrong here, but think that another couple where leaving (inside) whilst we sat outside, no-one else arrived during our meal. As for our reception meal it did seem exclusive, however maybe the rep at the hotel can confirm if the restuarant will be open to others. I'm unsure if you'll want it set outside or inside (I highly recommend outside obviously), so presumable it may be a case based on numbers that they may open the alternative area to hotel guests, once again I would say confirm with the hotel rep.
It is rather secluded, as it is set at the end of the hotel, not far from the beachfront. There is a walkway at the front of the restuarant boundaries that did have a few people walking from the hotel to the beach for a little stroll.
We went in May (23rd was the date) and it was a little windy that night, so it did get a little cooler, so not sure how October will be compared to that. We also found the hotel to be very quiet and laid-back, and even at night at the bar's the place wasn't packed with guests.
Did you say you where staying in the hotel? What rooms did you book?
FD