It is back. Despite digging it out a few weeks ago (a 5 foot square hole!), I thought I was rid of it, but a trip to the allotment today showed a small little leafy growth in the area not yet planted.
After breaking off a leaf and rubbing it between my fingers, it was definitely horse radish. The damn thing was back. This small piece was lying flat about 2cm under the surface with the shoot end poking up through the compacted soil surface.
There were a couple of other small leaves growing further along and they were also attached to small pieces of horse radish I missed the first time round.
They have been removed and now I will need to keep a special lookout for any more.