Twirlypoo, DH is self employed and was sent a very similar request and ours has all been sent back with no changes to our Tax Credits, so I may be able to help.
Firstly, try not to panic - if you have kept HMRC informed of any changes then it should be a simple process.
Summary/diary of bookings: We enclosed invoices which detailed the hours worked. DH did not have a diary of bookings as such, but he had regular work and invoices requesting payments for almost every week (see below for reasons why a couple were missing).
Invoices: We were actually missing a couple of invoices from City Link (DH subcontracted to them before they went into receivership). City Link used to create the invoice for DH and he then sent them to payroll, but obviously in the latter weeks there were massive problems within the company and this fell by the wayside. So we submitted bank statements highlighting the payments from City Link and explaining why some invoices were missing. Obviously, they understood and, as City Link are no more, we cannot do anything to generate the invoices. We were also missing invoices from the hire firm DH uses for his vehicle, despite requesting them numerous times! Again, we just highlighted these payments on the bank statements.
I imagine, if you enclose bank statements and highlight the payments, they may be happy with that. Either way, they will get back to you if they need further clarification.
A breakdown of hours worked each week: This one is going to be harder to overcome. Basically HMRC want to know that you have been working the minimum hours weekly in order to qualify for Tax Credits. If your hourly rate works out to being below NMW, they are going to want reasons why. So for example you work 25 hours per week but, by the time business expenses are deducted, your hourly rate is closer to £5.50ph, then they are going to be very suspicious.
Registration documents for vehicles: Send whatever documents you have and request new registration documents from DVLA urgently.
As for using your business account as a personal one, if it is a proper business account, then you really shouldn't. I would go through all your statements and highlight the business transactions.
I enclosed a breakdown on a sheet of A4 for each month, detailing income and expenditure (with both broken down further) and also enclosed all the evidence I had to support that.
Please do this ASAP and send via special delivery - HMRC will work with you if you communicate with them, but if you ignore them they will stop your payments. Good luck.