visual studio vs visual studio code for unity

(This is why I do not want to move from Unity 2019.3.14F1 to 2020.whatever at the moment - you never know what might happen). I am wondering if anyone has met this before and knows how to fix it? I am not very familiar with the workings of VS Code, but I assume it must be reading the Unity project file to do intellisense etc. I realise this is almost certainly as VS Code related problem, not Photon, but Photon.Pun is the only bit it objects to. The problem is VS Code thinking there are errors. The code however is just as it was - it does not come up with any compile errors in the Unity editor itself, and it all runs fine, including the networking Photon parts. The error says 'the type or namespace name 'Pun' does not exist in the namespace 'Photon' (are you missing an assembly reference?)'.

The instant result of this is that there are all sorts of compilation errors showing up in VS Code relating to my multiplayer code interfacing with Photon. Stupidly, I took Microsoft's advice to update VS Code, and its version went to 1.50.1. The VS Code version was 1.48.3 - and everything was fine - no compile errors, all code working OK etc. The Unity version is 2019.3.14F1 - I know there are later, but I don't want to move forward just yet in case of 'unexpected problems'. I am developing Unity game apps on a PC using C#, running Visual Studio Code as the editor.

Visual studio vs visual studio code for unity