'The bit I'm really struggling with is the people who do the line "oh, so you're a magician…" and then expect to see something immediately. This is particularly a problem when someone who knows the stuff you do introduces you, so they have seen it all before.'
This can be a common problem. A lot of people have never seen close up magic before so they will jump at the opportunity to see magic first hand. One thing you should always do, whether you show them magic tricks immediately or don't show them anything at all, is to be polite. If you are polite and likeable, it doesn't matter what you say.
If you are serious about your persona as a magician then you should always account for these types of situations. Have a few select items in your wallet or on your person so you have something to show. Turning around and saying, 'oh, I haven't brought anything along with me' (unless you are a stage magician!) is a major let down and immediately ruins any mystery you may have about you.
If you are introduced as a magician, and someone wants to see some magic, remember that you are still in control of your actions. If you don't show them a magic trick right away, it doesn't have to be a bad thing. Sometimes delaying your performance and not showing them something right away can help intensify the excitement when you are ready to perform, which in turn can lead to better reactions.
Some of my favourite magicians are always 2 steps ahead of anyone else. Magicians like Gary Jones and Greg Wilson will already have loaded a card in your pocket or set something up as soon as they settle into a situation. If you pre-empt situations like this, when you do perform some magic tricks, it will seem absolutely impossible. If you keep this in mind and ready yourself, that is half the battle. Whenever possible, try to predict these situations!
However, if you really don't want to perform you should not be afraid to say that you are having a night off. Make sure you tell your friends that there won't be any magic tonight, and ask them not to introduce you as a magician. Most friends will understand this, and won't want to knowingly put you on the spot.