Transfer domain auth code

I have started a .co.uk domain transfer from fasthosts to ovh and so I changed the IPS tag in my fasthosts control panel.
I have since received an Auth code from ovh but I'm unable to use it.
I think the reason is related to the fact that I already have a DNS zone file configured with ovh for this domain.
Initially, when I started the transfer with ovh, I searched for the domain name, it came up as transferable but I skipped the step for entering the Auth code as I didn't have one. Now that I have an Auth code I can't find where to enter it. I don't understand why if ovh sent me the Auth code why they couldn't have used it for me to complete the process.
Hope somebody knows what I'm talking about and can help 🤪
Dave