Ah right, thanks. So if I've understood OK I would:
1. Move my current wallet from my laptop to my desktop
2. De-seed it using the above code
3. Follow the instructions at
https://electrum.org/tutorials.html#offline-mpk from the "Master Public Key" line onwards.
Is that right?
Yeah. Though the easier way might be what ThomasV just said:
1. Click "Master Public Key" on your wallet.
2. Copy it.
3. Click "Create New" under File.
4. Name your wallet file "default_wallet" (or you can just name it something else, then rename it later, Electrum opens default_wallet by default on startup)
5. Choose "Create New Watch Only Wallet"
6. Input your Master Public Key from #2
7. Delete your old seeded wallet (assuming the seeded wallet is already on your other computer) and make sure the new watch-only wallet is named default_wallet and is in the wallets folder.