@Paffman
The problem that you've run into is that the Big Value Bundle is a bundle which you actually buy, unlike the Freebies, where you get a bonus of minutes or texts or data when you top up, and you get to keep the top-up.
So, yes, it's not totally unlike having a monthly contract, except that you can take a break whenever you want, you don't have to pass any credit checks, and your credit rating is totally unscathed, and there are no penalties, if you decide to stop tomorrow.
What you actually need, from your description, is indeed a Freebie - see the Freebies tab here - so that you top up, you get your Freebie allowance, and then you start eating into your credit at standard PAYG rates. The trick is to switch as effectively as possible, preferably without accidentally buying a further BVB before you switch.
So, is your original £30 BVB still running? If so, you should be able to opt out of automtic renewal, either via your online My Vodafone account if you've set one up, or by calling 2345, and fighting your way through the IVR - which is, of course, heavily influenced by the product(s) you have at present, but if you make as though you are going to choose a new BVB, it should offer you the opportunity to opt out of renewal. Then wait for the BVB to expire, opt in to the Freebie which you want, and only when you are confident that your opt-in has been accepted, top up by at least £10 at one go, and you should activate your Freebie.
If your BVB has already expired, you should be able to opt in to your chosen Freebie, wait for confirmation of the opt-in, then top up.
As long as you didn't top up by a further £30 before the original BVB expired, you should be OK with either of those schemes of action, as you won't get a lower-value BVB unless you specify that you want to swap to that. But a top-up made before you've opted in won't trigger a Freebie, so you don't get the "bonus" allowance on top of your credit.
Sorry, it isn't particularly easy to explain. I've been doing it since the year dot, and it's a bit like falling off a log for me, but I still struggle to explain it coherently. If you find yourself in a situation I haven't covered, please provide details, and I'll see what I can suggest!
I presume your previous experience of Vodafone is on a monthly contract - PAYG is rather different, and I think that's so for all the networks. I seem to remember seeing a comment from you a while back, about lack of phone signal in rural Norfolk? None of the networks are great - well, there just aren't enough of us in the rural areas - but things are gradually improving, and 4G creeps slowly towards my neck of the woods.
