I’m goign to have difficulty sinking an anchor to secure my bikes but I’ve hit upon another idea. My intention is to build a small box in the shed and pour cement into it with the requisite bits of ironmongery (4 or 5 very robust bike stands I think, anyone know where to get them?).

I know a shed isn’t most secure but I figure that cement is a decent thief stopper. Is there a flaw in the plan that I’m not seeing?

sounds like a lot of cement…

how about cut a hatch in the floor, dig a hole and then fill that with cement with some kind of anchor that comes up above the floor level when you refit the hatch. Then lock to the anchor…

how about a shed shackle, i got my bike stolen a year ago, put in a shed shackle. a few week later the tealeaves came back but left it alone when they saw it was locked up
have a look here

http://www.torc-anchors.com/shed-shackle.php

I did something similar as a ‘next best thing’ alternative to a proper ground anchor. A large bucket filled with concrete, with an old D-lock cast into the top to form a projecting loop for a chain/cable through the bikes. Its pretty heavy, so anyone trying to nick the bikes has to carry the bikes and heavy concrete bucket.
Obviously not 100% thief proof, but as with all anti theft measures, if it makes things harder or slower, it’s better than nothing.

I totally forgot shed shackles. Time to get ordering.

I had a large steel rectanglur hoop made up in the heaviest section of rebar the local supplier had in stock 40mm i think it was. I dug a large hole under the shed and set the rebar loop in 1/4 ton of concrete. The rebar hoop stuck up through a hole in the bottom of shed. The shed was broken into a couple of times but the bikes remained safely locked up.

Landmines.

tried the land mines but the neighbours complained everytime i let the dog out

There was a previous suggestion of a starving wolverine, spray painted orange and fed LSD. I can’t find a wolverine in Surrey sadly.