Does your ISP offer hourly breakdowns of data use?
You may want need to check usage figures and log them your self
i would do it like this
1. Change your Wifi password, and also the admin password, and any other password for your modem/router
2. check usage then, turn the modem/router off when you go to bed, turn it on in the morning then check usage again, is the data usage still high?
3. check usage then, turn off all devices or put on air plane mode, turn them on in the morning then check usage again, is the data usage still high?
4. check usage then, turn off all but one devices or put on air plane mode, turn them on in the morning then check usage again, is the data usage still high?
5. repeat the above till you find the culprit
or download data monitoring programes for your devices, i use https://www.softperfect.com/products/networx/ for my windows PC's and https://play.google.com/store/apps/details?id=com.roysolberg.android.datacounter&hl=en on my android. but android has a pretty good one built in in the settings.
record and monitor values when you go to bed and when you get up. analyse and then you might have your answer
Thanks guys. Appreciate your suggestions - they are helpful. This one looks good - https://www.softperfect.com/products/networx/.
I will try that.
As some of you pointed out - Vodafone customer service could do much better than what they do right now to investigate problems like this. Their bandwidth meter shows usage for whole day - even hourly break-up may help sometimes. Really ISP should make it much easier for layman users to provide information like this even if it involves bit of traffic monitoring for few days. Anyways. Things will improve - I am hopeful and positive!!