Quick answer
The referral credit is not usually instant. Octopus says the new customer needs to be on supply and their first monthly Direct Debit needs to clear before the referral reward is paid.
Why it can take a few weeks
The switch starts
Octopus confirms the supply switch and contacts the previous supplier.
Your supply moves
The account has to go live before the referral reward can be completed.
The first payment clears
Octopus waits until the first monthly Direct Debit has been successfully taken.
The credit is applied
The reward is added to the relevant Octopus accounts after those conditions are met.
What if the credit has not appeared?
Check that you used the personalised link, signed up directly, completed the switch and have had the first Direct Debit taken. If those are all true, contact Octopus with the referral code lotus-book-642.
Octo's missing-credit checklist
Before contacting support, write down the sign-up date, the page or link you used, whether you used a comparison site, when supply started and when the first Direct Debit cleared. That gives Octopus the useful facts in one message.