I would like to add two double sockets and a light to the garage. The
garage is attached to the side of the house. Currently a spur has been
taken off the back of a socket inside the house to provide a single
socket in the garage.

I understand the correct way to do this would be to run a separate
circuit from the consumer unit to the garage to feed the double
sockets. You would then take a fused spur off the same circuit to
provide power for the lights?

The only problem with this is the consumer unit is full and an
additional extra mini consumer unit has already been added for a
shower. Is it time for a new consumer unit? Alternatively would it be
feasible to extend the downstairs ring main into the garage instead or
would this be classed as a bodge? The area covered would not exceed
the maximum area for a ring circuit?

Any help is appreciated.