How to create a hedgehog hole in a fence with concrete in the bottom?

I'd say 1 hole would be sufficient for that number of panels ( unless you are expecting an army of hogs ).
Also worth remembering that Hedgehogs aren't that fond of Slugs and Snails - they will eat but apparently only as a last resort.
Hope it works out and you get some visitors.

 

Thank you so much Philippa I'll stick with the one hole then. Not expecting an army though how wonderful that would be!
That's good to know about slugs and snails, as I have had quite a few snails around but haven't seen much evidence of them being eaten.

 

I agree, that first photo would be the ideal spot. Could you (a) poke a stick underneath to see if it hits anything solid and/or (b) get a stepstool or similar and poke your nose over the top of the fence to see what's over the other side?

 

Thank you Lizzie I'm glad you think it looks good. When I look underneath that wooden part on the left without concrete, there are snails under there so it looks like there is possibly something blocking underneath, I will try poking, and I'll try having a little look over the fence tomorrow too.