and finding loads of *** type lumps of stuff, any idea what on earth this is please?
Apparently the houses are build on the land that housed a former furniture factory if that helps.
Just looked back and see that Melv mentioned clinker. Honestly had not seen it. Try not to read other answers when making my own mind up, but I have raked out a steam engine fire box and it looks just like that. If you also find what looks like chalky flakes in the same place it is probably lime scale from the inside of the saddle tanks or boilers and makes it nailed on.