If I'm reading you correctly, it looks as though the sump is silted
up from the surrounding soil. If that's right, then to make it work
as a soakaway again you'll have to dig it all out and refill with
good lumpy rubble: maybe it needs to be a lot bigger, too, to stop it
happening again.


I think I would go along the same lines, BUT, before putting lumpy rubble
in, I would get one of those perforated drain tubes, used for land drainage,
say a metre long and drop that into the hole the put the rubble into that.
This way the soil is less likely to silt up/clog up the 'workings'

I think you can get them about 300mm diameter

Mike