There is some ‘Big Bud Mite’ around the allotment
Big Bud Mite is bad news for blackcurrants, it causes a disease called reversion which eventually stops the bush fruiting. You can minimise damage and spread by removing the ‘big buds’ now.
If you look closely at the bush you will see healthy, slightly elongated, normal looking buds – any larger, more round buds have the mite in them. You can pick off these buds now – DON’T COMPOST them – put them in landfill or burn them. Otherwise as soon as it warms up they will release the mites who will stay in the soil till next year. You probably need to consider rplacing your blackcurrant bush in next couple of years as they sicken.