As said, it's down to different labour rates due to places having less staff and overheads to cover.
The first quote of £950 is a piss take, maybe they over quoted because they didn't want to do the job, some garages do this.
I'm not sure how many hours Suzuki quote for such a job but I would have thought around 4 hours + parts
The set figure of £570 if it needs shims or not sounds good.
If you go with them, I would ask for the measurements of the gaps and what shims were charged (if any) for future reference.
Every time I do shims, which is rare these days , I take photos of the Bike apart and show the customer plus, on their bill, the gaps and what shims were changed.