From their terms and conditions
Termination of Contract by LRTT
In case LRTT has declared a specific training program to be subject to a minimum number of participants, LRTT may terminate the contract if such minimum number is not reached.
In such case, customer will be notified two weeks before course start at the latest. Any training fees paid by customer will be refunded.
Any other claims will be excluded.
The same will apply in case the training cannot be performed due to force majeure or other reasons for which LRTT is not responsible. Notification will be made without undue delay.
Next write a letter saying that if payment is not received within 7 days you will be taking legal action to reclaim the outstanding amount.