Where do you live?
Guessing by the fact you're quoting 2007 I assume England. I'm a landlord in Scotland and this only just came into effect for any new rents after 2nd July 2012.
Anyhoo, there's only a couple of approved schemes in Scotland:
http://www.lettingprotectionscotland.com/
https://www.safedepositsscotland.com/
http://www.mydepositsscotland.co.uk/
So I assume it's the same case with England. As such I think you might have to check each one has no record of your deposit.
Also note there are a couple of reason why a land lord does NOT have to put your money in a deposit scheme. For Scotland, these are:
Lets to family members
Life rents
Houses for holiday use
Properties used by religious orders and organisations
Accommodation with care
Houses subject to control orders
Agricultural and crofting tenancies
Resident landlords
Transitory ownership (executors, heritable creditors, and insolvency practitioners)
If you don't fall into any of those categories this might be the reason why you can't find any information about your deposit.
Regardless, your landlord SHOULD be telling you what has been done with your deposit. I can't comment with any certainty how it will be viewed that you've not paid the full deposit but I'd imagine its the kind of question that a deposit scheme would be able to answer pretty quickly. Maybe the same email to each of the English approved schemes would yield quicker results and give you piece of mind?
I found the websites for the Scottish schemes to give me more than enough information as a landlord and I've had no questions form any friends, family or tenants regarding the scheme that weren't answered on their websites or by email.
Good luck!