Thanks for all your help and advice.
Lucid - I got the SIM a while ago, bought a £5 goodybag, used that up, and then of course was left with £0 credit. Now I need to top-up again (as I will definitely only need to use £5), I thought I'd go for the Lab as it seemed like the perfect option, so I paid £5 for that on it's own. If I'd have realised I'd have topped up with £10 credit, then purchased the £5 Lab afterwards!
Unfortunately I need the credit today so I'll just have to top up £10 I guess... don't have time to wait for points to add up. Like the idea about promoting Giffgaff at the event though! But unfortunately we have a deadline by which all the money needs to be in, which well before December so not sure that would work out.
Thanks though for your suggestions!