At first, I thought it was ceramic tile bit as I dug down further I realised it was an enamel sign. It looks like a previous owner used it as shuttering when doing the concreting around the shed. What little I can see of it seems in surprisingly good condition. I hope it hasn't been cut down to fit the job.
I followed the line of bricks along and lifted the slabs and it seems there's another sign in there.
I can't get them out yet, as we were putting in a temporary patio area for a hot tub. I'll be making a better and more permanent base if we decide the location proves favourable this summer. Either way, all being well I'll be digging out the signs in the autumn.