On their website it says,
"✓ Pay for your 2020 break just 3 weeks before you go
To give you more time to pay for your holiday during these uncertain times, we’ve moved the balance due date for bookings* with arrival dates up to 31st December 2020 to just 21 days before your holiday is due to start. If you choose to cancel your holiday before your balance due date, then we will waive your liability for the remainder of the balance and no further amount will be collected from you. If you choose to cancel on or after your balance due date, then you will need to pay the cancellation charges (according to the number of days before the start of your holiday) as set out in the cancellation charges table which you can find in your booking conditions, as well as any applicable booking fees, admin charges or insurance premiums that you owe. See booking conditions here."
https://www.hoseasons.co.uk/covid-19
That means you need not pay the balance until August.