hi all i need some advice concerning an extension wall we have lived in the house for 16yrs and the previous owners had a single storey extension built 18yrs ago over the past cople of years i have noticed a crack where the extension wall meets the back of the house so i decided to strip the plaster back to have a look and it looks as though the wall isnt tied into existing brickwork and there is a 15mm gap all the way down any advice please about what i should do thanks