Please see the following provisions relevant to you:
"6.4. Storage & Structures
6.4.1. It is important to seek Committee approval for any structure before proceeding,ideally at the planning stage. As a general rule the following dimensions should be used when planning: -
Sheds should be no more than 6’x8’x8’ on full and half size plots; sheds on quarter plots should be no more than 4’x4’x8’.
Polytunnels and greenhouses (which must be fully glazed) should occupy no more than 20% of any plot and not more than 8’ in height.
(Metric equivalents apply)
6.4.2. Irrespective of size and including siting, no structure can shade/impair adjacent plots formore than a quarter of daylight hours.
6.4.3. Allstructures must be safely built and contain no hazardous materials such asasbestos or other currently prohibited materials. They must be properly securedto the ground, but bases and footings should never be made of concrete.
6.4.4. Only materials necessary for the cultivation and maintenance of the plot can be stored on-site in sheds or other structures. Members must not bring on-site or deposit any domestic or other rubbish. Any items brought to the plot must be for agreed use and any unused or surplus materials must be removed as soon as possible.
6.4.5.Members must not, nor permit others to stay overnight or reside in sheds or other structures."
I would advise you to comply with their request as you were in breach if you did not seek prior permission.
All the best. Please leave a positive rating.