Presumably this would be a witness statement BUT I would ensure that that is exactly what it is and what it is going to be used for.
You can always ask for the legislation under which the meeting is going to be conducted and whether it is voluntary or otherwise. If it is voluntary then the choice is yours and of course you are not obliged to provide any prosecuting authority with information if it is voluntary. Of course what you have to ensure is that any information you provide does not breach your confidentiality with the client and therefore while there would be nothing to stop you speaking to documents and as to your knowledge of the documents/transactions you would have to be careful that you were not providing "what you thought" the client's knowledge etc was of these or the implications. Remember if it is a witness statement it is your statement and not the officers so that if you don't agree with what is being suggested you can just refuse to prepare it on that basis.
Presumably if you are still acting for the client you have absolutely no reason to suspect or consider that there are or have been activities that would result in Suspicious Activity Reports.
You also have to consider whether HMRC have suspicions that your firm could be implicated and I would be asking the officer if he was suspecting that you had intentionally held back information given his/her question after having seen all your correspondence on the client's system. In that event you would be best advised to seek your own legal adviser to discuss matters with before committing yourself to any particular line.
regards and hope this helps
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