23 is too warm for me ideally...... at work we have temp wars in our offices. i would happily run at 18 degrees with a jumper on (not with aircon to get it there of course) but as soon as it goes over 21 i start to get sleepy. at one point it was 27 degrees a few months back and that was horrendous.
Personally i think a good base temp to aim for with all other things equal and no energy crisis is 20 degrees....... its warm enough that even nesh southerners should be ok with a jumper on, and tough northern vikings should be able to cope with it that warm too