The shock on my dad’s 2007 Trance has gone very soft. Last week it wouldn’t hold pressure, this week he was running around 40% sag with 210psi and he weighs under 12 stone. Is a service likely to cure this or is it likely to be something more. Shock has not been touched since new 🙁

Service time I’d wager.
You could try doing an aircan service first. That seems a little high pressure-wise.

Service time, as oil & seals will have been well ‘worked’ so probably leaking in various directions.