Hello OH,
The Sundial story is an old one..with apologies to anyone who has worked on Eastbourne foreshore..it is only used as a representatative location in this case??!!...
I have seen the same sundial come a cropper as a result of a Henchman work platform being pulled with the person, nameless, walking backwards...I can't talk as with the very same platform I tipped over a large tin of black bitumen paint.

...I was only being helpful too..moving it for those who were struggling with it...their brushes were quickly put to use brushing up paint off the york paving and working it back into the tin..
I also admit to on occasions just clipping the side of a low Box hedge with the big mower and the ride on seat...the old Ransomes Mastiff was good at this game as it was such a powerful brute with lots of pent up energy that took an age to steady down if a manoeuvre went pear shaped....
As soon as it caught the hedge it would tend to spin it into the hedge and you had to try to hold the handlebars straight against this whilst try to get to the throttle lever to shut it to tickover...
I have looked behind before now after seeing bits of Box going through the blades and expected the hedge to be neatly trimmed to 2 inches high..
I tried out a newer version of the Mastiff 36" a few years ago at a local cricket club..it is much safer???..as it has the throttle as a bar that you hold up to the handlebars for full throttle and lower it for slowing down and tickover...I tried a tight turn to simulate going around our trees..I suddenly found that I couldn't release the throttle bar to slow the thing down as I was gripping the handlebar to execute the turn.!!....the seat started to tip and I nearly exited stage left..off the seat..!! Anyway the mower would have stopped if I had fell off..not like our old one when the seat pin dropped out...
The seat would deposit you on the lawn nose first whilst the mower headed at full throttle for the nearest border....or Skegness..!!!
The old Atco sounds like it may have the cutting cylinder gone out of true...in that it is cutting on some cylinder blades but missing on one...
If it needs to be set dead tight to cut then it sounds ready for a cylinder grind and a new bottom blade to match...the cylinder bearings should be checked for wear and replaced, if suspect, during the strip down/rebuild.
Alternatively it could be the bottom blade deflecting. When a cutting cylinder is set too tight to a bottom blade it tends to deflect the bottom blade down as each blade on its spiral sweeps along the bottom blade..this too leaves the effect you describe.. and gets worse as a ridged wear pattern develops on the bottom blade.
Some of the older Atcos and I suspect the more recent 20" Special, the 24" and the 30" are quite fiddly to set as if I recall it involves two screws/bolts being adjusted that push up to a fulcrum swivel block thing

rather than a bolt or nut being tightened with a spring opposing as used on Ransomes Marquis, Matador, Mastiff
Sounds like a job for a local service agent.. they may be a bit quiet at the moment praying for a wet spell.. whilst mowers sit silent in their sheds..
All the best,
Clive.