Yes, you will need to notify them even if the inverter remains the same. You originally had 2.55kW of generating capacity installed so would not have reached your inverter's rated 3.6kW output limit, but with the extra panels you will now be able to achieve the full 3.6kW and the DNO will want to know about that change as it affects the amount of export you will be able to achieve, although still within G98 limits.

There's a threshold at 3.6kW / 16A, below this follows one process G98 which is typically connect and then notify with this being permitted, above this is G99 and you typically need to apply and get permission for these larger capacity installations.

More details on the ENA website - these are the standard processes most of the industry follows to give you some guidance. You should also look at the website for your local distribution network operator to see exactly what their processes require.

https://www.energynetworks.org/industry/connecting-to-the-networks/connecting-generation-to-the-electricity-networks

a big thank you to you all, especially transparent who is a star