Hardly. I'm a plumber by trade myself.
Not all Worcester boilers are that expensive, and if you think they are then I'd suggest speaking to your merchant. Even a quick google of retail prices reveals that you can get them significantly cheaper, although granted we have no idea exactly what model it is so either one of us could be nearer the mark.
Plummerman has it spot-on, some great advice.
Yes. Again, you paid for a day's work and the knowledge, not the actual time spent doing the job. I really can't be bothered to go through all this again, though.