Amalgamation approved - where to start first?

[Amalgamationproject]

Hi, I am looking for a bit of advice as I am not sure what’s the best order of things here.
I have two flats in a converted house which I am converting back to a single house. I have obtained planning permission and now I need to combine the titles. One flat is owned outright and the other is with a residential mortgage.
I have read that I need to fill in a form AP1 for the registration of the combined title but do I need a letter from the bank first to apply for the combined title or do I need the combined title first to change the mortgage?
Also, do I need a solicitor for the AP 1 form registration or is it a straightforward process and no solicitor is required?
I am speaking with the bank today but any advice from people who have done this will be greatly appreciated. I’ve received great advice here before.

You will need a solicitor. Some of this can be done yourself but the bank won't agree to only mortgage half a property, so you will need to move the mortgage onto the entirety of the new title (or repay it) and that will require solicitor's involvement.

I assume the two flats are on leasehold titles and you also own the freehold? The Land Registry can merge the titles and will close the leasehold titles.

Thank you for your response. Yes, I also own the freehold title for both flats. I will speak with a solicitor as well then.