I would aerate it well, and apply compost regularly. Perforated drainage pipe in grit at the bottom of the slope will help, also, as long as there is somewhere else for the water to go.

Drains should be placed at the lowest part of the slope and then drain away from that and any buildings. On top of the drain larger stones then medium then smaller gravel on top in layers - am guessing about 40Cm/1.5 Ft in depth. It sounds like the soil is compacted as it is running into your property.

This should help and is what I have at the bottom of my slope and around my house. You could put soil on top of the gravel but I used decorative gravel to keep the drainage free and put some terracotta pots on top to make a feature.

Thankyou so much for your advice, I'll certainly give it a go. Glad I found this site, good advice is well worth having!