You can sue the ex tenant but you need to know their full name and where they live now. If you don't know then you could ask a tracing agency to find them - most offer a no trace no fee service.
Here are a selection of tracing agencies you could approach :
Once you have their address I would recommend that you send them a formal letter before action to demand payment within 14 days and say that if they do not pay you, you will issue county court proceedings against them. See attached template as an example of what to say. Please let me know if you cannot see the letter which I have uploaded. You will need to tailor this letter to your situation.
You will need to register at https://www.moneyclaim.gov.uk/web/mcol/welcome so that you are ready to issue the claim in the event they dispute the claim and do not pay you. The website is very user-friendly and you would not need a lawyer to use the money claim site.
Claims with a value of under £10,000 are classed as a "small claim", so legal costs are not recoverable and the matter may be dealt with on paper by a Judge, not at a hearing. This means the parties are on an equal footing, so you don’t need to worry about legal costs if you lost.
A hearing may be necessary if the court thinks that oral evidence is required to dispose of the case.
Here is a user guide for the money claim online site: https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/762843/mcol-userguide-eng.pdf
The court will then issue the claim and they will send you "notice of issue". The papers are served on the defendant who then has 14 days to acknowledge the claim - they do this by filling out an acknowledgment of service and they post it to the court. They indicate their intention when they do this, i.e. whether they admit the claim in full or partly, or if they deny the claim. If they want to defend the claim then their defence is due by 28 days from the date the court served them with the papers. The central court processing centre then sends the claim to the defendant's home county court for case management and directions - the directions will give a list of dates which you both must comply with. If there is no settlement then the claim will be dealt with at a final hearing which takes anything from 9 to 12 months from when you start the claim - longer if the claim is higher value. You can pursue the claim yourself or use a law firm. For the hearing you can use an advocate if you wish, though it's not compulsory. I have details of law firms and advocacy agencies if you would like those. Though in a small claim you won't be able to recover their charges from your opponent. A small claims hearing is easy to do, it's quite informal and no lawyer is required.
You would claim the sum for the loss, the court issue fee which is £35 based on your claimed sum and court interest which is 8% calculated on a daily rate from the date of loss to date of court judgment.
The site allows you to calculate the interest and add it to the claim. If a hearing is required then there is a fee for this too - see page 7 of the previous link for details. Again, that fee is recoverable if you win.
If you are on a low income or have low savings (or in receipt of benefits), you can ask the court for a fee remission so you do not have to pay the court issue fee.
If you are eligible, let me know as you cannot use the online money claims site – you have to use the paper method, which is an N1 claim form sent to the County Court Money Claims Centre (I have a copy, let me know if you would like one).
If you win then once you have the CCJ from the court the defendant has 14 days to pay in full. If they do not then it gets registered with the credit agencies after 30 days. You can also enforce the CCJ with the county court bailiffs if they ignored the court order.
You may find they just pay you after receiving the letter before action – hopefully they will want to avoid litigation.