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Depends. First Class on the Sleeper for about £110 gets you a cabin to yourself, and breakfast. I only use the sleeper occasionally and as a result tend not to sleep - beiing aware of every bump, stop and clackety clack. Seasoned veterans claim to sleep soundly and arrive well refreshed.
If your alternative was LHR I'd be tempted by the sleeper - due to queues and atc delays - but on the whole LCY tends to be efficient and flights generally leave on time. Though there is no room for delay!