I'd say £420 was very cheap. Maybe if there were no "extras" at all, but I always find that new brake pads, new sprockets, or, perish the thought, new opening rockers
are needed.
I've just had a look at my last major, done about a year ago. Pure "service" parts (oil, filters, cambelts, spark-plugs etc) came to £157. Basic labour charge was £170 so yes £420 sounds plenty. But at the same time, I also had the radiator stripped and cleaned, the headstock bearings replaced, one new rocker fitted etc etc.
I won't tell you what the final bill was
but I'm not for a moment saying it wasn't justified, it certainly was.
Where are you getting it serviced? I use louigimoto, who's just outside Bristol, and is a top man. Just a nice little run from Plymouth.
As for what's involved, it's all in the Haynes manual. Do get one if you haven't already. Even if you don't see yourself doing much DIY servicing, it's still twenty quid well spent.