First of all I'd like to take the time to welcome you to the forums and thank you for your post.
With regards to using the Call divert feature on your line, you'd be correct in saying that it will use the free calls within your talk plan.
For example, if you were on Talk Weekends, and you diverted calls to your Virgin Media Mobile phone number, then on a Saturday and Sunday this would be free of charge, however on a week day, these would be chargeable.
If you were on Talk Unlimited and diverted calls to a mobile, then if it were a Virgin Media Mobile phone then it would be free, however other networks would be chargeable.
For more information on Call Charges, why not check out This link.
Thank you for the clarification, thats not what I was told by the online chat whilst i was changing to virgin mobile (they said all calls would divert without charge), but at least we now know the position.