Issued with a court citation against my business (Scotland)

As title says I’ve been issued a court citation over historical debt. Took my eye off the ball and buried my head in the sand as business has been tough. I’m in a better place at the moment and slightly better finances. The debt is with a water company. The letter was had delivered by Royal Mail and I have 21 days to reply if I want a solicitor to represent me. . . I have no argument against the amount I owe so dont see how representation can help. I have phoned the company directly and engaged with them and offered to set up a payment plan. My figure and there figure are miles apart as they want 50% payment up front which i cannot afford at this moment in time. What are my options and what happens after 21 days? They are ‘suing’ to get recover the full payment.
Thank you.