Nav Bar – doing odd things !

Hello, can anyone advise …
I have just added a nav bar to my site, but when I click on each title, it messes up.

It’s hard to explain, but when you click among the different title on the Nav Bar, it displays the first page correctly, but the subsequent pages, it does something strange …

If you look at the web address (at top of the website) this is what happens…

https://webaddress.co.uk/product/
https://webaddress.co.uk/product/webaddress.co.uk/biography/
https://webaddress.co.uk/product/webaddress.co.uk/biography/webaddress.co.uk/links/

It seems to add the address of the next page clicked, to the first one …
Does anyone know what I am doing wrong ?