The only way that you can get a restrictive covenant removed yourself is to petition Parliament to pass an Act specifically to remove the covenant. This, obviously, is not a practical solution.
Your local planning authority has the power to disregard 'redundant' covenants. This would apply, for example, where an old house had a covenant relating to the provision of stabling for horses used by stage coaches. It's unlikely, however, that the planning authority would be minded to disregard the covenant on your house.
As it's unlikely that you'll be able to get the covenant revoked or disregarded, any changes to the garage would be in breach of planning rules. This means that the council would be able to force you to reinstate the garage to its former state.
Note that the convenant seems to restrict the use, as well as the structure, of the garage. If, for example, a market trader was to buy the house and use the garage to store his goods, the council could insist that he removed them.
Chris