I work for said company, and if you have a record of the IMEI you can record it as stolen and we can blacklist it foy you. We do take note of the imeis when they are shipped however, you have got your phone through a dealer and they have swapped it, they may not have updated the system and would have the k750 one listed instead. If thats the case, theres not a lot we can do. We can block imeis when they are switched on but we can only find out what the current IMEI is against a number, not what it used to be.
Your best bet wouldve been to contact Voda and say this is the imei number of the phone that has gone missing etc. Surely Phones4u have it on an invoice somehwere? If it was scanned, then it would be on a reciept? This might help!