Hello
If you have already signed a contract for the period you are being invoiced for, this is binding on the council for the period of the contract, and the rent cannot be unilaterally increased unless there's a clause in the contract saying it can (I'd be surprised if there was in an allotment contract but it's worth checking).
If the invoice is for the next year, the council can increase the rent. The new rent should be stated in your new contract and cannot then be increased until the next time...etc...
£23 sounds extortionate for an admin fee just for giving you a contract and I've never heard of a council charging for this, but there's no legal reason why they can't and no limit on what could be charged as far as I know. You could try complaining and asking them to justify the charge in the light of the very minimal amount of work involved in filling in your details on a standard form.
Good luck!