We've got our list with JL and they do give you cards to include but decided not to use these as they're normally the first thing to fall out of the envelope and I didn't want that with our invitations.
So, we just put a short line at the bottom of our information sheets - which had directions, hotels, contact details etc on as well. We fussed a lot over the wording but in the end we decided to just be straightforward about it - people will buy a gift if they want to, they won't if they don't, so we just wrote:
'Should you wish to buy a gift for sdaisy22 and H2B, they have a gift list at John Lewis, list number 123456. The list is open for 8 weeks from 14th September. You can buy a gift online www.johnlewis.com, by phone 01234 567891 or in store.
I think it was also a bit easier for us to word it like this as the invitations came from my parents, not from us.