I have never heard of Plum before and have done some googling:
It's not backed by the Financial Services Compensation Scheme.
It says: "FSCS was created to protect consumers against the risk posed by banks lending out their deposits. ... Savings with Plum are held as “e-money” in protected bank accounts protected by the electronic money regulation. Because Plum is not a bank and savings with Plum are held as “e-money”, FSCS cover does not apply to us."
I have no idea what that means. If they are taking deposits I would think they should be in the Scheme.
Per Money saving expert:
"Plum deposits your money into a wallet operated by MangoPay, an EU-regulated financial institution. Any money you save with Plum is held in a Barclays instant-access savings account. This means that if Plum were to go bust, you’d be able to recover your money from Barclays.
However, if Barclays were to go under, your money may NOT be protected by the Financial Services Compensation Scheme, so you could lose your cash – you need to decide if you’re happy to take the risk."
They are also, as far as I can see, not regulated by the FCA. This is the latest info I can find:
"Plum is currently not independently registered with the FCA, but in the final stages of applying for a registration as an Account Information Service Provider. ... Plum works with regulated payments institutions, and is itself registered with the FCA as an appointed representative for two of its partners." (info dated March 2019)
Again, I'm not at all clear what that means. I personally wouldn't touch it until it got it's independent registration.