Hi andy,
hope your well mate,
Yup pretty sure its stone, its not that particular house but its the closest pic i could find, We get quite alot of them in this neck of the woods I guess their must be a big hole somewhere near by!
Do you think high pressure will the mark the stone block like it does on the (sand?) stone lintels you sometimes see on modern houses?
Will have a go at the acid on a test brick but ideally dont want to use it because there are quiet a few houses to do and i think it might be too time consuming. would you spot treat with the acid or do a wash with it?
steve