your up in the lake district arnt you otto ? if so there slate aint to bad and it is local coniston way i think the quarrys are , slates deffo better optoin than concrete but it all depends on costs on your part keep us up dated otto
does look like a new roof to be fair , the only temporay solution i can think of is replace any broken slates and replace the flashing might work for the short term
not 100 % sure but all structual work by your builder should still be under garentee so my advice would be to get him out tell him you want a structual engeeners report done . Do you know if the steel was calculated?
And do you know if it was put in place on pad stones ???