I tried to send my Bitcoins to my wallet earlier but have managed to miss the last two digits off the address when copy/pasting. The payment was sent straight away but has obviously not arrived, is there anything I can do? I have a horrible feeling I just lost all of my money to a typo, any help will be gratefully received.
To include details from your comment: you were trying to send coins from your account at some sort of online marketplace.
Losing coins because of an address typo is generally not supposed to happen; however, preventing it requires that the marketplace has designed their software properly. The fact that it got as far as it did suggests that they have screwed something up. Whether the coins are actually lost is something you have to find out from them.
It will help to explain a little about the Bitcoin address mechanism. A standard Bitcoin address (1xxxxx...) has encoded within it a 160-bit public key hash, and a 32-bit checksum (actually computed via another hash). When software wants to send coins to an address, it should normally:
decode the pubkey hash and checksum
verify that the pubkey hash and checksum match
generate a transaction sending out the desired number of coins, listing the pubkey hash as the destination (the checksum is not used after this point)
post this transaction on the Bitcoin peer-to-peer network
wait for the transaction to be confirmed.
In order for the recipient to eventually spend the coins, they have to be in possession of a private key whose corresponding public key has the same 160-bit hash as contained in the address.
Step 2 is where your typo should have been caught (barring extremely bad luck; see case 1 below). After this point, no more checking is possible: there is no way for anyone to determine whether a given 160-bit string corresponds to a private key, unless they know what the private key is. I would think any reasonable software design would perform step 2 as soon as you typed in the address, display an error, and refuse to proceed until a corrected address was given. The fact that this didn't happen is a bad sign; it suggests that the marketplace's software is not well designed. But we don't know what it actually did do.
So there are a few possibilities for what happened:
You were extremely unlucky, and your typo'ed address decoded to a hash and checksum that matched each other. The probability of this happening randomly is about 1 in 4 billion. You can check whether this is the case by entering your typo'ed address in a block explorer site like http://blockchain.info and seeing if you don't get an error message. In this case, the marketplace may have gone through the whole process described above, and sent coins to a hash that doesn't correspond to your private key. The transaction would be valid and been confirmed, but in this case the coins are now lost and nobody will ever be able to claim them. If so, the marketplace should be able to give you a transaction ID for the transaction they created; you would be able to independently verify that the transaction exists, was confirmed, and corresponds to your erroneous address. There isn't anything the marketplace can do to avoid this case (except maybe asking you to type the address twice).
The marketplace performed step 2 at a later stage of their processing, and discovered the error in your address. In this case, they never created a Bitcoin transaction; they still have your coins, and should be able to re-credit your account. Ideally they would do this automatically, but you might have to ask.
The markeplace decoded a hash from the address you gave, but never performed the step 2 check, and instead created a transaction sending coins to the incorrect hash. Like in case 1, the coins are now lost, and they can send you a transaction ID which proves it (you can compute for yourself what the incorrect hash is, and verify that the transaction sent coins there). Unlike case 1, this case would clearly be the marketplace's fault, for not performing the step 2 check which would have caught the error. The right thing for them to do in this case would be to refund your coins out of their own pocket. Of course, they may or may not actually choose to do that (especially if they are "shady").
The whole thing was a sham; the marketplace made it look like they were sending you coins, but in fact had no intention of actually doing so. There isn't much you can do in this case except to try to pursue legal action (good luck with that).