It's dawned on me, our proposed garage will have a hipped roof with some south facing roof space, approx 7m x 2.5m. It will be slightly over-shadowed by the house itself from the east. Is it still worth considering fitting PV there? 

possibly - will work better in the summer when the sun is higher. You could price micro inverters so that even if some panels are shaded, the rest produce full output, otherwise without optimisers you could affect the whole garage string? Or setup two smaller strings with one being furthest away from the house if the inverter has the spare connectivity? 

We have looked at doing this As with putting PV on the main house it will take many years to cover the initial outlay It seams like a good idea to add PV to a garage roof Rather than spoil the look of the house 

Is putting the modules on the walls an option.  You get better winter output, your electricity delivery curve becomes flatter, instead great in summer rubbish in winter.