Do I need to install a local ground point for a secondary consumer unit in my shed?
From your reference to "socket ring", I assume you are in the UK.  I shall use UK terms.

From the additional information you have provided in the comments, you have the easy case.  There is no long run from house to shed, so you don't have to worry about resistance in the cable reducing the short-circuit current (either live to neutral or live to earth) to the point where the fuse in the consumer unit doesn't blow.

Just use the earth wire in the cable, and export the earth from the house.  Do not attach another earth in the secondary consumer unit in the shed.

Do make sure that there is an RCD in the circuit - either in the secondary consumer unit, or at the point where the spur comes off the ring-main.  (Personally I think where the spur comes off the ring main is best, but in the shed is certainly "good enough".)

Do make sure that the spur to the shed is adequately protected by a fuse.  Note that ring mains can go up to 32A, despite the fact that they are wired in 20A cable - this is because they are only rated for 32A on the assumption that it is reasonably evenly divided over the ring - so each leg of the ring shouldn't go above 20A.  With a spur there is only one leg, so you can't rely on that - you need to limit it to 20A.  There really ought to be a 20A fuse in the FCU where the spur comes off the ring, but if the spur doesn't come off the mains in an FCU and it's difficult to install one, bunging a 20A fuse as the master fuse in the secondary consumer unit won't be strictly according to the regs, but it will get you most of the protection.