Oil pressure gauge problem

[Sterly]

Hi, the mechanical oil pressure gauge (Smiths PG1502/02 - presumably original) on our 408 sits at 20 instead of the hard stop at zero, engine not running. With the engine running it goes to somewhere around 80. I thought that the needle might need repositioning or maybe some light oiling inside to free things up. So far I've only got the glass off but haven't been able to remove the needle so I can reposition it. And how do you get the internals out of the case? There are 3 screws on the periphery of the gauge, near the oil supply tube so I assume these need to be removed but I haven't ventured that far yet!

Thanks in advance
Mark