Sorry for any confusion around Big Talk. The tariff is explained in more detail here.
You just need to top up during the month and your rewards will be applied on the 1st of the next month, you don't need to top-up again before the 1st of the month - as long as you topped up £10 or £15 the month previous you will get your rewards regardless of however much credit you have left - however you do need to leave a bit of credit remaining to cover the cost of a chargeable call/text/data usage - even though you won't be charged you need to leave a buffer to be able to use the free allowances.
The top-up's don't roll over - to get the rewards on the 1st of each month you need to top up £10 or £15 every month. If you don't, you don't get the rewards.
Hope all of that hels, however I'm unsure what you meant when you said: "At no extra cost’ must mean there is a charge being taken from the £10, but I can’t find info on this." ..can you clarify what you mean?
I'm guessing that number is charged premium rate? 789 worked fine from France a few weeks ago, now it only give the credit amount, then the call ends. Before, I could talk to Virgin staff. Are you able to contact Virgin themselves, ask why 789 no longer works from Europe?