I'm not a tax expert, but as I read it you're only liable for Corporation Tax on any profit. If you're a "not for profit" organisation, then you're not.
I think your link answers your question: allotment societies aren't liable.
In an allotment society, any surplus from one year is carried over to the next for possible future expenditure. We pay no salaries, nor have no shareholders; all "stock" is held in common.
In our case, we're a limited liability company, so all members are shareholders - to the tune of £1.00 - in case we ever face bankruptcy. (It does mean that we're regulated by the FSA and have to fill in an annual return, but that's fairly straightforward.)
Contact NSALG - OK, you already have! - and the FSA.
Are you actually a private company limited by shares and not a private company limited by guarantee?
In what way are you regulated by the FSA? Companies House I can understand, but not FSA.
As I see it not-for-profits still pay CT because not-for-profit just means dividends aren't paid to shareholders and even charities pay CT, but it's the line in the "who doesn't pay" that says "allotment and garden societies" because it also says that limited companies incorporated in the UK do pay CT, so if you're an incorporated allotment and garden societies do you pay or not?
I expect I'll have to ask HMRC, but I'm sure they won't give me a simple answer.