You were fleeced. The travel agent gets commision for selling extras like "pre-booked" seats. The term "sitting together" is a very loose term indeed. You can have the isle between you and still be classed as "sitting together". The only way to garentee sitting together is by contacting the airline (First Choice special needs telephone number is 0870 757 2757) and requesting specific seats. There may be a small fee involved but not as high as the travel agents fee. The other wa is to arrive nice and early at the checkin desk. Flights are very, very rarley 100% pre- booked. They are more like 40% pre-booked.
Again call the airline on the above number with regards to the ski's