I think the Waitrose trailers are old news now. Some stores have scrapped them due to 'lack of demand'
It might help if they had advertised the service.
It's worse than not advertising. The only way to find a store that has them is to ask at individual stores. I saw one in use, and asked at my local store, and they had never even heard about them.
I realised later that it was probably not taking groceries from the Richmond store, but returning the trolley to Twickenham, but I never saw any sign of at either store.
If you search waitrose.com for cycle,bicycle or bike you get no hits, but it's still mentioned on this page
http://www.waitrose.com/content/waitrose/en/bf_home/bf.html but in two years, they haven't added it as a store locator option