Hello Alanst, if you are currently a pay monthly customer with o2 any for any additional line you will receive a family plan discount. If you have a local store it will be a good idea to pop in and they will be able to add the additional line onto your current account and walk away with the sim on the condition you pass any relevant checks and credit checks. You can also get your pac code from vodafone by texting the word PAC to 65075 and they should send you your PAC code within 1 minute and have it transfered at point of sale subject to taking upto 48 hours to transfer, vodafone may charge you any remaining months you may have with them.
Hope this helps.