Interesting techniques for surveying / garden design
Designer Bunny Guinness has just done a few videos on DIY surveying techniques and gadgets - getting accurate mapping and levels.
You may be able to find your garden plots / allotment on the Ordinance Survey map using the OS app, and print it or copy the scaled images of your garden. OS offer a one month package for £3.99, which could be interesting to try for surveying purposes.
The OS are currently also offering a one week free trial of the app here, which might be good for a one of image grab of your plots to scale. Just be very sure to put the cancellation of account / payment / direct debit into a day of your diary so pay nothing and don't fall into the trap of laziness falling you into paying long term for an app you don't use. I haven't done this myself, so I don't know the magnification you can get to, but it might be worth a try.
This might be interesting to see what the OS have on record, for plot boundary questions.