I have got the door frame but find there is nothing to fix it to as there is a cavity void. What is the best way to circumnavigate this problem?
you need to fix aluminium tie straps to the the door frame leaving them long enough to fix to the concrete block wall which needs to be pre-drilled with a masonry drill bit !insert sufficient rawl plugs and screws making sure that the frame is perfectly level and plumb!depending on how high your door opening is 4 fixings on each vertical frame is sufficient!best of luck!
I have two friends in the building trade. There called bodgit and leggit.