One of the conditions of any airline booking is that you accept THEIR Terms and Conditions which are easily accessed from the airlines' home page.
According to Jet2's T&C's, EX CARGO CLOWN should have been offered a choice of three options when Jet2 decided not to operate a flight after a booking had been made.
1. Transport to destination 'AT THE EARLIEST OPPORTUNITY ON ONE OF THEIR FLIGHTS'
2. Re-route to destination 'WITHIN A REASONABLE PERIOD OF TIME ON ONE OF THEIR FLIGHTS OR MUTUALLY AGREED MEANS AND CLASS OF TRANSPORTATION'
3. Full refund.
If EX CARGO CLOWN has not been offered ALL THREE options then he/she has valid grounds for complaint, unless the prefice to these options 'Except as otherwise provided by the Warsaw Convention OR the Montreal Convention OR EC Regulations' provides JET2 a legal get-out clause.