Andy - your cokin P filters should work with a new lens but you will probably need a new adaptor eg if your current lens is 52mm filter thread then you will have a 52mm adaptor, if a new lens is 62mm then you will need a 62mm adaptor - they're only a couple of quid. No need to get a new holder.
Regarding the second part of your question - if you're not sure what effect wide-angle has then how do you know you need one? Please excuse me - I'm not being flippant here. If you need a definition/description then have a look
here HTH.
regards