we have a soak away drain that after a heavy rain shower or prolonged rain doesn't drain very quickly.
I understand how they work and that they don't normally clear as quickly as a mains drain. But in the last few months it seems slower. Is it as simple as digging out the silt that has built up or should I try something else?
Basically dig it out and rebuild.
You probably want to do some porosity checks on the ground around the area first and see how it performs... best doing this in a wet spell to make sure it is going to cope - then size your soak away using the official guidelines for physical house size rather than how many people live in it to save any future headaches.
Most groundworkers should be able to do the work for you. Porosity testing is a bit of an arse, but not overly complicated really and the calculations are pretty straight forward.
And don't use perforated, coiled and ribbed pipe. It may be "made for the job" but it is rubbish. Just use standard 110mm soil pipe and slot it with a grinder 😉 And use fresh materials rather than the old stuff.