You wont be able to erect a building using agricultural notification rather than full planning due to your holding not meeting the minimum 5 hectares (about 12.5 acres) for Class A rights under the Planning Act. Unless you have more land within 5 miles on the same holding number.
http://www.legislation.gov.uk/uksi/1995/418/schedule/2/part/6/made (NB for any size holding, stables/horsey stuff is not agricultural and so needs PP anyway, not relevant for your query but for others reading. eg we qualify as have 40 acres but still needed full PP and building regs for our stable block).
You cant put up any permanent building as such without getting PP for it. The only option for the lambing might be some kind of mobile thing, maybe a lorry or something! But hay storage, the only option might be an old fashioned haystack :-)
If you want a building for hay and lambing then you probably need to work with the planners/neighbours to come up with something that is workable, it may require spending more ££ on security/CCTV etc if it has to be somewhere screened.