Ouch! £30 for half a dozen inserts. Hammer and block sounds like a much more economic alternative. Having not done any insert replacements I can't speak from experience, but I did read in one of the Vid1900 posts on Pinside.com that using a clamp is good as it holds the insert in place until the glue is dry. Sometimes if you level with a block the insert might move during drying. One problem I can imagine with the hammer method is that if you do more than one at a time (and they're close to each other) they might shift as you tap another one level.
Eventually I'll be a dab hand at this as I want to make a new playfield from scratch for PARAGON, so I'll be doing 56 inserts altogether! I think I'll rig of some kind of large press/jig so I can squeeze a block of inserts all at the same time. No wait... thinking about it you don't level new inserts... you leave them proud then sand them flat with the playfield. Mmm...