Errr... I'm sorry to be the bearer of bad news - I'm afraid it's not that simple.
If I were you, I'd be getting on the phone to a housing lawyer for a consultation, and possibly to draft a letter to the tenant, depending on their advice.
The problems are thus: you have 99% now lost your ability to evict this tenant. If your tenant gets wind of the fact that there are rules around tenancy deposit schemes and specifically timing, they could apply to a court not only to get their original deposit back, but also three times that deposit on top. And on top of that, you even still may have a very hard time evicting them if things go sour.
The last thing I would do is fling the deposit back to them, because it's weird, and your tenant may google 'why has my landlord given my deposit back?' - which will almost certainly lead to links to 'deposits must be protected within 30 days.'
Now, I've had two tenancies in 2 years. One of them definitely sent me an email notification when the letting agent put it in the scheme, and I don't think the other one did. There are three different tenancy deposit schemes, you see. I am 95% sure that whatever one we're on now did not send ME an email, but if I look online, I can see it's been protected.
The lawyer might just advise you to protect it now and hope for the best.
They might advise you write to the tenant, apologising and explaining, saying that it is now protected, but leaving out the 'you are now entitled to your money back and the same again x3 if you chose to issue proceedings.'
Did you give your tenant a 'how to rent' leaflet? I might be wrong, but I believe that's a legal requirement now, too.
A court will not take a house move as reasonable justification for not being on top of admin. They may take ill health as an exception, especially if it can be proven.
You could call Shelter (best time to get an immediate answer is 8.01am), but they're very pro landlord, and might not really want to give you advice, or maybe they won't even know what you should do, but they are free, and (as a tenant), I've always found them to be really helpful.
I'm sorry about your health and I hope you now have a lovely home and are on the mend.