Hi,
as the house has been rented for 10 months, it will be more difficult to remove tenants if they are the ones not paying.
The first thing you need to know is, who issued the cheque that bounced. If it was the letting agent, then you need to meet the tenants and tell them that you want to be paid directly and not thru the letting agent. Also ask if they have a receipt from the letting agent for rent paid.
If the tenants issued the cheques that bounced, then you need to instruct the letting agent to start the termination procedure immediately.
The agent should have organised the PRTB forms and registered the letting for you. You can still register and pay a late fee which you should do.
At this stage i think you are at damage limitation and probably wont get the money owed. You need to try to resolve this as soon as possible so as not to lose any more money.