I have no experience with this particular scenario and can't tell you exactly how this particular Spanish consulate will treat your application but in theory it should not be an issue.
As far as the regulations are concerned, you should have a legitimate purpose and the financial means to pay for your accommodation, nothing more. There is absolutely no obligation to have each and every night accounted for and technically, you don't even need any specific booking or reservation, although it can be useful to support your application.
Your other question suggests you applied for a visa for a longer, more complex trip. If you do get a multiple-entry or two-entry visas from October 1st to the end of your trip to Greece, it would also allow you to enter on the 11th so the whole issue is moot (there is no such thing as a visa valid for one entry on the 1st and one entry on the 12th, it's either a single-entry visa or two entries at any time during the period of validity).
Since you are, as far as I can tell, a UK resident with a good profile, my guess is that you will get a multiple-entry with enough flexibility not to worry about this. But even if you don't, you would still have the option of shortening the trip at the other end (returning to the UK one day earlier to avoid going over the maximum stay limit).