Image Cruising refuses to to reimburse refund

My wife and I on 14Feb20 booked a Danube river cruise with Imagine Cruising, which was to be hosted by Jenny Morris (the giggeling chef) on commencing 26Aug2010, at the same time we paid a 50% deposit of R46,000. On 26Mar20 we cancelled the trip on genuine medical grounds (having just had a double kidney cancer operation), given we would be unable to take out insurance as they do not pay out on pre-existing conditions. This cancellation was done 156 days (5.2 months) before departure giving Imagine Cruises ample time to sell this trip elsewhere. In the meantime, Covid-19 struck and RSA went into lock-down 27Mar20, this being after our cancellation. On 6Apr20 we received an e-mail that they were not accepting our cancellation and that we would still be liable for the balance of R46,000 to be paid by 14May20, furthermore they were imposing a total charge (being the deposit paid) of R46,000 as a penalty. Understandably you can appreciate that we were loath to further throw good money after bad, and have not paid the balance. We also tried to attempt a charge back on the Credit Card to no avail, as Imagine Cruising were refusing to re-imburse. Now on 19May20 they claim that they offered to replace our bookings for next year. We have now requested if the identical cruise, hosted by the same chef would be available at the same period in 2021, and are awaiting a response.