Just trying to find out if I can undo my one shot nut with a torque wrench that has max 210nm, prob a silly question but I don't have £150 to buy a 300nm wrench, I don't know anybody to borrow one from either., I know the setting is 290nm.
Firstly, a torque wrench is for tightening nuts to correct setting, it should not be used for undoing nuts or it will go out of adjustment. You use a socket & long bar to undo nuts but to tighten to correct torque you will need the 300nm wrench. The wrench you have is not sufficient for job.
The question that you have asked is so basically wrong that it makes me wonder if you have the knowledge to service your brakes?
There's always more than one solution to a problem.
For example, if I happened to be 60kg, then my weight due to gravity would be roughly 600N. So standing on the end of a 0.5m long spanner would create 300Nm torque.
That's standing, not jumping.
Anyway, I weigh a little more than 60kg, so I'd try and borrow a small teenager.
I wouldn't even consider spending £150 on a specific type of torque wrench, not when I can get a full caravan service for £120. The service engineers have the right tools for the job plus they know exactly how to set up the brake shoes and linkages properly. I have a basic torque wrench solely for emergency purposes in case of a puncture but one shot nuts etc. needs to be left to the experts with professional equipment.