Puddytats - what a nightmare!
You need to think very carefully about this.
I assume that the cheques in question were cashed???
If that is the case then the person who cashed them must be someone very very close to you and your DH.
To open a new bank account today all banks require at least 2 pieces of identification. These include passports, drivers licences, council tax bills etc. Whoever did this must have had access to these documents for both of you.
Who might that be? Does his secretary keep his passport when he is not travelling - does she pay bills for him?
Is there anyone else in the company who may have had access to these documents??
They will now look closely at whose accounts these cheques were paid into, where the account was opened etc.
The bank will probably have cctv coverage which can be checked if it is was opened at a high street branch. If it was done via internet banking then there is more of a problem, although the IP of the computer used should be able to be traced!
A set-up only works if the person has been able to open the account and cash the cheques- you need to think hard how this might have been possible!