Heard a radio advert this morning - not sure about T&Cs but the gist seems to be that O2 will match your top-up amount for the time being (Until end of Jan. maybe?).
Top up by €20 - they add €20 etc.
I think max. top-up is €50: to which they will apparently add €50.
Tesco are doing the same. (they use the o2 network) double credit on all top ups 10 euro or above. The extra credit expires after 30 days and can't be used till you use up the credit you paid for.