Thanks for the replies/info chaps
Turns out to be rotting moss
I just being quite careful as there is a lot of rubbish in this garden built up over 100+ years, lots of old tools, car bits, tyres, platics, metal, loads of brick, slate, victorian glass bottles, crockery, everything you can think of, including asbestos tiles.
Even found an old rear mech
will have to clean it up, the chrome work is still quite bright.
I think it was an old pigary, with a dung/rubbish pile behind it.
The tiles are getting bagged ut and the council will take it then.
EDIT: Garden isn't that big tbh, and the fergy is borrowed from the father in law, he drove it down for me, nearly 30 miles away
It's a joy to ride, makes short work of it all.
I've also had to move 7 toads so far, you can just see the soil start to move on it's own and head pop out, poor things, but there is plenty of other undergrowth for them to hide in where i've been putting them