Good observation Craig, in hindsight it may be possible to remove the backing plate first, before the pulley, then remove the circlip on the axle. That way you have a flat surface across the pulley (bit of wood or something) to sit your clamp for the backing plate squash.
Then remove the axle after removing the vane housing and vanes. It is the snuggest, perfectly engineered fit so even with a minute amount of oil it seals and kind of bonds, so the gentl(ish) persuasive blows can help.
Hitting the nut above the pulley with the axle still in the housing shall not make a difference at holding at a low height (besides dropping and damaging the housing). The circlip would pop regardless.
Take note that the axle, my guess it is chrome vanadium, is very tough (ever tried cutting chrome vanadium steel?!). Don't have to worry about damaging that groove. The circlip is not something heavy duty, but would be easy to replace, it does not come in the kit. In my opinion the kit is not great value for 2 rubber o rings and a copper crush washer..if you measured them you could probably source them much much cheaper individually. It should be 3-5 bucks rather than 30 odd, genuine merc rubber and copper or not..
For the most part, the circlip holds two pieces in place whilst assembling. Once the entire unit is assembled the circlip only plays a minor role, as the large spring holds the vane housing in place. I consider my circlip ok still.
Both pumps I have from w123's (zf 1984 230e and vickers 1985 230te) start with the numbers 126. I suspect the radiators from 123's are the same as 126's too.
"And yes i have been tricked with the inline thing, puts the new one at top not bottom.... but you will only make the mistake once!"
are you referring to the inlet and outlet connections here? Yes, it makes sense that the inlet is the reservoir. Internally in the pump housing, the exit is a larger slit shaped hole.