Hi Mark, I`m also in Norwich,
When I was looking for a shed about a year ago there were lots being advertised in the local free ads paper and I even saw them in shop window adverts.
Although they were free they had to be collected so you will need good transport. I have none. They often needed to be taken down too so if you need help with that I`ll give you a days labour, just post here when you are ready.
Having a shed made a big improvement to the way I could use my plot so best of luck,
Columbus