I have finally managed it.
1. Aligned UCS with end profile (circle) of CHS.
2. Using that UCS, rotated cleat using the centre line of the edge beam as a reference angle.
3. Then aligned UCS to cleat. Then used ALIGN command to align the cleat with CL of edge beam.
4. Used UNION to make rafter and cleat 1 object.
5. Cleat is now aligned on CL of edge beam and also in line with axis of rafter.
Seems to work - thank god. Big thanks for all your help!