I have a wall that has been covered in soil on one side higher than the damp course by 600 mm for 20 years the mortar appears to have crumbled on that side. Advice please.

how big is large ?
if it is over 1/2" but under 1" cut a slice of tile to fill the gap
if under 1/2 inch use a silicon sealer ( no more gaps) to fill the space
to get a smooth finish on the sealer use your finger --wet it in water or suitable applicator that is wet with water
once the desired finish is achieved , let it dry and it becomes water resistant

Hi, have you tried T S P concentrate for the wall treatment.

If you have the room you can install a "french drain" outside your garage wall. A drainage pipe or pipe and barrel covered with crushed stone. You might also be able to waterproof the outside wall of your garage foundation/slab.

More information is needed.
Brick wall is inside apartment... what is on other side of wall?
Open existing electrical box. Drill through back of box and drill through wall until you come out on other side of wall. Install new box #1 on wall where drill comes out. Run conduit over to another new box#2. Drill through wall again until you come out on inside brick wall where you want to relocate original box. Install new surface-mount box#3 on brick wall. Then run wiring from original box. Original box cannot be bricked over or covered with building materials since it is a junction box where wires connect.
Other option is to install extension ring on top of existing electric box. Run conduit across face of brick wall. Install new box#1 where you need outlet.
Other option is to chisel away the brick and mortar from existing electric box down to floor level (or over to door frame). This will allow recessed conduit that can be concealed with matching mortar. Then run conduit across to new location. Cover conduit along floor with baseboard, or chisel out brick along floorline to accommodate conduit, then mortar over conduit. Or simply paint conduit to match brick. You must use conduit since wires cannot be run openly.
Gene

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

1930 where 26,262 were made. http://www.mchenry-sc.org/Engineering/Model_12.htm

Your fears are well-founded. Here in NW, MO the sand and mortar are heated prior to using in freezing weather.

Check all round the window and ensure a good seal with the brick and sill [use silicon sealant at the joint between the window frame and the brick & sill and brick] and that the sill has a drip bar [ridge or channel under the sill running sideways at the furthest away from the wall] If not put one on the underside. Almost anything will do [sounds silly but an elastic band will do temporary or even a line of the silicon] so long as it collects the drips from the sill. also ensure that the sill slopes downward & away from the wall
How old is the extension ?? I ask because if it is 50+ yrs you may have lime mortar which lets the bricks breath and if this has been re-pointed later with non- lime mortar [i.e. portland cement mortar] any water which gets in above will not escape out through the lime mortar below.
Secondly, if this is a north or east facing wall getting little sun and if the bricks were originally not good, frost & freezing will cause the flaking.
Bricks - has the wall been built with facings or commons. commons will spoil [flaking] sooner.
Do your bricks have a frog. That is a hollow in the middle [& looks a bit like a cake which has sunk in the middle] The frog should be facing down otherwise water will collect You can only check this by taking a brick out [applies mainly to modern bricks ]

Pictures of the exact item along with any markings would help a great deal. Even a hand drawn sketch along with dimensions would help.
It sounds as if you may be describing a scope cover. One end cap is clear and the other end cap either clear or has a yellow tint.

Most all current covers have the end caps attached to each other using two stretchable thin bungee like cords although some do have a one piece rubber holding the end caps.

If this is what you are looking for any sport shop, most discount stores and such carry them.

I attached a picture of this type of cover.