As with everything, Baz... it depends.
I'm assuming access is good. A ladder can be put up to gutter height, and a roof-hook ladder can be safely set onto the ridge.
For a start, I would re-bed rather than re-point. Re-pointing could easily fall out again unless you're very lucky. It really wouldn't take a lot longer to re-bed. The worst bit is shinning up the ladders with buckets of heavy mortar.
One chap on his own would re-bed in a day. £200-250 should cover it. As for looking for broken tiles... probably best to let him deal with them as he finds them, on an hourly rate... as an extra.
If ladder access is a problem (outbuildings/conservatories/extensions etc in the way), then it would get more like the £450 you've been quoted. If scaffolding is needed, then much more expensive.