Gave my 30 days notice to 3 and asked for my number to be changed to Pay as you go
Looks like it has just changed but for some reason I am still able to use data, checked the 3 app and it says I have no allowances left and instructs me to top up, is this normal?

When your using data is it not charging you then?

Did you top up your PAYG account at all ? If so normally Three provide 150MB free with each top up and it lasts 30 days. So even if you run out of credit during this period you can still use what's left of the 150MB fee internet. Maybe something along these lines.