I discovered yesterday that someone in the Philippines has been synchronising their phone into my gmail account. I think I know who is responsible but I am not sure.

I then changed all my account details, including a different phone number, and my password and had verification from Google that all the changes had been made.

However, this morning I see that the account has still been synced by the same device

Does the notification I receive from Google indicate that the syncing was successful or merely attempted?

The system just keeps telling me to go through the same procedure again to change everything.

Thank you in advance for any advice

They can't sync to it if you changed your password. If you get a notification of a sync with an error, then you are ok. I would change your secondary notification email address as well.

Is someone using an old phone of yours?

If so, remove the device from your Google account.

Its an interesting question.

Firstly, I am not offered an option to remove it - quite why I dont know. Maybe because it wasnt me that put it there in the first place. My new phone does have a remove option

I was recently in the Philippines and I did have a phone stolen similar to the one syncing into my account. It had a Philippine SIM and in the cover was my Thai SIM. I have checked the model number shown on my Google page but when I check it out, that model wasnt launched until a year after I bought it, so unless there is a mistake somewhere it isnt my stolen phone.

After sending my first message in this forum, that phone is shown again (at about 2pm ) to have synced with my email account and not showing an error so it seems to me that the syncing was successful. All this in spite of me changing my password and my new phone number yesterday.

If the Google Services are enabled, you can use Device Manager to locate, ring, lock or erase the account of the device.

Android Device Manager

You can also use this Google link to review your current account security settings, especially the links under Recent Activity - Devices.

Google Account Settings / Data Tools