The clerk has no powers to hire and fire staff, unless the council has previously delegated this function to the clerk. Do you ever compare the cost of direct employment with the alternative of hiring a specialist contractor to undertake this work? I presume that you are also providing your staff with the necessary equipment, which adds to the cost.
Whilst this arrangement of making staff redundant and reemploying them may suit the council, I would advise you to seek specialist HR advice. This practice may fall foul of the continuity of employment regulations, giving your ground staff full employment rights. Failing to advertise a vacancy could put you in the firing line for a breach of equal opportunities legislation. For example, is there an appropriate gender balance amongst your ground staff?
I'm not an HR expert, but one alternative might be to clarify their employment status by employing them on a permanent zero-hours contract, giving you the option of varying the start and finish dates of the main season and carrying out odd jobs during the winter months.