And been through the whole process with an operator on Google call back.
[Very friendly and great customer service, BT could learn a thing or two (or ten) there!]
We took one of the tablets and eliminated all the other possibilities: Google Play cache corrupted, Account broken, Download Manager not working, date and time not correct and so on.
What we found is that our devices will all update on other networks and the phones will update over 4G.
It's just our when connected to our home hub that they don't update.
I'm so glad it's not just me. Thought I was going mad here with this issue. I have a HH3 and I'm having exactly the same problem. It suddenly started the latter part of last week and none of our android devices will download from Google play store, over wi-fi. But those that are data enabled work fine over 3 or 4g.
I hope this is something that BT are aware of and are looking into a fix because i have tried messing around with a tablet and now it needs a full update and noone can download anything on the tablets now.
Thanks to those of you who quite rightly identified that the issue is in fact the Parental Control.
It's madness that it takes customers to resolve these problems rather than the provider.