Hi Martin,
We’d always recommend that it’s best to come to an arrangement that works for both parties wherever possible, although in this case it does seem you have acted in good faith only to be let down.
The most common solution in these situations is that the tenancy is kept going until a suitable replacement tenant can be found. The original tenancy is then mutually surrendered and the new one set up to start the next day, so there’s no void period.
It’s a bit of work, and there’s no obligation for you to agree to this while you’re in the fixed term of a tenancy, but as @katherine81 mentioned, having a tenant in your property who doesn’t want to be there does increase the risk of damages, late payment of rent, non-payment of rent, abandonment, etc.
Please do keep us up to date as things progress!
Best of luck,
Sam