It may be a lot of money, but I doubt they are ripping people off if it is £45 fitted. Replacing a crystal on a watch is not a simple job. Typically the watch will have to be dis-assembled and the old crystal removed, then the new one has to be fitted, then it has to be tested (presuming the watch is waterproof). This isn't a 5 minute job like replacing the battery in a quartz watch. Given the labour hours involved and - very often - the specialist tools and materials, £45 doesn't seem unreasonable. Generic crystals can be bought which can make it cheaper, but many manufacturers (and I believe Seiko is one) restrict you to their own make. That is why it is sadly often not economic to repair a watch with a broken crystal.
willie_gunn