If the judder started when the clutch went light then something doesn't add up because it's hard to see how a failed flywheel could cause the clutch to go light. Do you trust the garage is the question I guess, sounds like they may be trying to talk the job up. Why did they decide it needs a new master cylinder, is it leaking, failing to operate the slave correctly because the seal has gone ? If the master had gone then this would explain the lightness. Smells a bit fishy to me, all these parts going at the same time.