There are big clumps of Comfrey on the allotment. They may be Russian Comfrey (Symphytum x uplandicum), which is a naturally occurring hybrid of S. officinale and S. asperum. It grows wild and is a well-travelled variety with an intriguing history.
 Comfreys were known by names like Bruisewort and Knitbone because their leaves used as a poultice were reputed to have such properties. The leaves are also a rich source of nitrogen, phosphorous and potassium which has long interested gardeners as an additive to compost and for steeping in water to make liquid fertiliser.
 I believe the name Russian Comfrey stems from the nineteenth century when it was imported from Russia by the Essex smallholder, Quaker and organic pioneer Henry Doubleday. His work was continued by Lawrence Hills under the auspices of the Henry Doubleday Research Association and he developed various strains, notably Bocking 14 which is still widely grown today.
 Perhaps some previous allotmenteer planted it as a green manure but Bocking 14 is sterile (i.e. can only be grown from root or crown cuttings) whereas this one springs up all over the allotment so it must be seeding. Comfreys are very prone to crossing and re-crossing with each other which suggests these might be Russian Comfrey going native or perhaps Common Comfrey (S. officinale) going Russian!   
 Anyway the biggest, fattest bumblebees are all over them...