Most permanent jobs in the UK usually have a probationary period written into the contract. This would usually be of six month duration, in some cases up to twelve.
So... with that in mind... The 'probationary' period argument for retaining flexicrew doesn't really stack up...
Hypothetically, if someone was brought in on a permanent contract and grossly underperforming then the company would be well within their rights to terminate that individual's contract - if within said probationary period.
Having said that, there's a difference between someone 'underperforming' in their role and being let go as they are just not needed over the Winter...
Flexicrew needs to disappear -
full stop.
Like any malignant growth, if it is not fully excised - you can expect it to flare up again in the future...