I agree, that plot looks to be bigger than 20' x 30', are you thinking yards rather than feet DT? Even 6p per square yard is expensive at 1.80/pole for water, but it is in the right ballpark - £16-odd per pole is ridiculous.
There are limits on how much notice the council must give for a rent increase, and sometimes the tenancy agreement doesn't even allow it. If you're interested in challenging it perhaps you could post the notice telling you about the increase, and the tenancy agreement too.