Hi Mr Blyth
I have worked on lots of soak aways and the majority of them had reached their life span having root or soil damage, back in the old days most soak aways were holes dug then filled with hardcore left from the house build. Nowadays soak away crates or 20mm shingle wrapped with Terram are used.
It's possible it could be blocked and you could have it jetted but 9 times out of 10 this would just fill soak away and sur-charge from gully.
My advice would be to investigate condition of soak away by way of a trail excavation.