So some background as to what happened here. I wanted to claim my bitcoin diamond air drop. I googled and this video came up first.
The video played this weird music which I think was hypnotic in some way. It instructed me to paste my blockchain.info restore phrase into this website: mybcdwallet.org.
I foolishly followed the instructions and did not realise at the time that I had handed over the restore phrase of my wallet to a hostile hacker. The next morning I woke up to find that a transaction had been initiated to transfer my bitcoins to another address (not owned by me!).
Fortunately the bitcoin mempool is clogged up with lots of transactions and the hacker used a small mining fee. Consequently the transaction had not been confirmed even though it had been initiated 10 hours before.
So what I did was installed Electrum wallet and restored my wallet into it. Electrum correctly assumed that I still had bitcoin in the wallet. I then basically did a "double-spend" i.e. I started another transaction of my own and sent the bitcoins to a paper wallet that I had created. I made sure that I used large mining fee to ensure that my transaction got processed before the fraudulent one.
The next morning I woke up to find that my transaction had been confirmed 13 times which means that I had taken back my bitcoin!
Lesson: Be very careful with your back up phrases. Don't type the phrase into a website you don't absolutely trust!
I'm not not sure if I can say the clogged mempool is a flaw of a design feature. But in this case it did save me a lot of money...