Late payment & disputing paid invoices from up to 6 months ago

I run a marketing agency for pet businesses. I have been working with a client for 10 months but just after terminating my contract due to not being able to pay for further work, he is now refusing to pay an outstanding invoice of £2777, due just over 30 days ago. I have just issued a further invoice for £2300 that will become due in ten days' time.

He issued to me a formal letter stating that both invoices would be paid on receipt of the following:

- The dates worked in full (8 hours as per your contracted rate) for all Invoices dated after 1 st January 2020
- Copies of all Third party Invoices for all items purchased on behalf of the organisation, using the personal card information belonging to the owner of the business. These are required for our company accounts.
- Detailed information of the tasks you performed on behalf of the organisation. Please break these down to daily, weekly, monthly and quarterly.

This letter also included the statement; "We have been transparent in that the given reason for the contract termination is simply that we need to perform some overhead recovery."

My response is that until the outstanding invoice is paid, I can not spend any more time working on getting these things together but that I will happily provide them once the invoice is paid.

My main concern is based around the questions they are asking about the dates worked and tasks carried out. Can they request details of all dates worked and which tasks were carried out when? It would take me forever to put all that information together and as they have now revoked access to the email account they have me to work with, it will be difficult to trace back times and tasks with any accuracy.
Also, can they query invoices that they have paid with no trace of concern up to now? I'm worried that not only will I not get the £5k that they owe me, but that they could also make claims back on the other invoices that they have paid.

I work on a daily rate that was agreed in advance and the client has never queried this.
My contract that they have signed makes no mention of having to hand over any files in the event of them terminating the contract, although I will, of course, once the invoice is paid. Am I wrong to withhold these items? I'm just concerned that once I have handed them over, They will have no incentive to pay and it seems like they have no desire to do so.

Thank you in advance for your time and I really hope that someone can offer me some helpful advice.

Hi,

But can they ask for hours and days worked from January and query invoices that have already been paid? It will take me forever to go back and track all the hours that I have worked since then....