Many crankcases are worn here...Just clean the damage up with emery cloth, give it a good clean afterwards and then refit all the parts with the additional spacer...
The 'spacer' machined as part of the engine sprocket carrier is the better option IMO as the whole thing is hardened thus reducing the chance of wear...
The separate spacers, which are not hardened, can develop a burr round the edge due to wear over time...
For that reason don't make the outside diameter of the spacer too close a fit into the crankcase recess, as if it does wear the resulting burr can contact the crankcase making it very difficult to get the spacer out at a later date...(and I've seen an engine that 'seized' due to the spacer becoming totally stuck)
I leave about .080" clearance over the diameter (.040" a side) and also put a slight chamfer on the edges of the spacer....Make the spacer I.D. a close sliding fit on the mainshaft...
If you have access to it you can prevent the chance of any further problems by making the spacer from flame hardening 'silver steel', hardening it after machining.. Harden it by heating it slowly to red and quenching immediately in cold water...Ian
email (option): ian@wright52.plus.com
There seems to be two 113s in the diagram.The main driveshaft is 113 &a component next to the shock absorber nut is also 113.
Its not April fool,honest.
If you enlarge the drawing enough you can see that the part by the shock absorber nut is actually (a not very well printed) 116....
That's the part Ron was referring to...As there aren't any parts in that location it probably is the spacer under discussion...
Somebody should send a telegram to the BSA drawing office and complain about the number and the location...
...Ian
email (option): ian@wright52.plus.com
Thanks all
I have managed to source a new sleeve with the 'spacer' attached.
Thanks for the support 
Ant
email (option): antkenmare@gmil.com