I have a domestic garage approx 6m from the house which at present has a flat steel sheet roof.
Its is also the best site for domestic PV, no overshading, existing electric spur from the house distribution board, easy access, etc

What I would like to know is :-
1 would I require planning permission to place PV units on this roof (free standing units designed for flat roof)
2 Would the fact that it is not integral to the dwelling influence the FIT rate.

(I am in Wales if it makes any difference

You don't need planning if you comply with the conditions for permitted development. See Class A as described here http://www.legislation.gov.uk/wsi/2012/1346/made - needs to be at least 1 metre from the edge of the roof and no more than 1 metre tall.

Assuming the electricity supply is common between your house and this garage then to get the highest rate FiT you will need an EPC at band 'D' or higher for the house. If it is a separate supply then the building may be exempt from needing an EPC.