I see neither the IAVI or the IPAV have a code of conduct when it comes to the treatment of a booking deposit by their estate agent members .
It would not surprise me one whit, based on past experience, if This ( unnamed) EA is using booking deposits to keep their cashflow up.
In the good times it always ‘showed up’ at the end…the end being maybe 2 years after it was handed over and with no interest accrued .
If you are asked for a booking deposit do not hand it over without a receipt and without a declaration of where it is to reside , the EA normally does not hand booking deposits over to the developer at any stage so make sure that you have details of how to claim the refund in writing and with a personal guarantee from an IAVI or IPAV member as to its safety ( for what thats worth ) .
If the deposit is to be transferred to the developer at any stage then RUN !!!