[Graham G3ZVT]

It very much sounds like he's done his best with a tube that has low emission on one or more guns.
If you want to try setting the greyscale up yourself, refer to the attached image.

Use an appropriate sized flat bladed screwdriver with an insulated handle.

Turn the colour down to zero to give a monochrome picture

The three potentiometers with circles adjust the overall brightness of the corresponding colour. Endeavour to remove any tint in the darkest parts of the picture.

The three pots marked with squares do much the same but affect the highlights to a greater degree, so use them to remove colour tints in the bright parts of the picture.

Maybe you can obtain a result that is subjectively more pleasing, but it sounds like the CRT is well past its best.