If you have got a few to get out, buy a breaker https://www.screwfix.com/p/titan-ttb280drh-15-5kg-hex-shank-breaker-230v/67819it will do the job in no time at all.
I do not wish to be rude because I think your product is a good one, but by the time the average joe public has bought the fixings and the undercutting tool it makes the cost too expensive in my opinion.
Why don't you cut a small hatch in the door that parcels can be posted through and secured with a Suffolk latch?
That way the gate would be secure but still allow for deliveries.
Mike
Have you tried searching for ' Victorian fireplace parts ' and clicking on the images tab? I have often found that taking this route to search means you can click on a image and visit a page to get what you are after.
Mike
Grass roots go down a maximum of 4 inches so dig over to that depth and spike the ground using a garden fork to allow air and rain to penetrate lower down .
Either that or just spike the ground over and lay turf....job done.
Mike
Out of interest do you have a doorway between the two rooms and if so is there a actual door or just some kind of archway?
The reason I ask is that for a door to be hung between two rooms, it usually means that a door liner has been fitted for the door to hang on one side with at least a four...
Just to give another perspective to this, have you had a chat with any neighbors to ask how their cupboards are fixed to the wall?
I would assume that if you are having to scratch your head as to how to fix them , then a neighbor may already have solved the problem.
Mike
Before anyone can give you advice, it's essential that you find out the background behind the plasterboard as in , is it brick and the plasterboard dot and dabbed on ?
Once we know what or how the plasterboard has been fixed to the wall we can offer fixing advice.
Mike
If you can persuade your customers that taking grass cuttings away is a bad idea ( cost +aggravation) then buy a mulching mower that will fit in your van and not be too heavy to get in and out.
Honda mowers are not cheap but they do last.
Mike