I know of a really good little shop that stocks practically any bolt, nut or washer that you may need. Can't remember the name of it but it's on the corner of Donald Road where it meets Boston Road, just off the Lombard Roundabout:
http://www.multimap.com/maps/?qs=mitcha ... ad%20(A236),%20Croydon,%20Surrey,%20England,%20CR0%203
They've always been really helpful in the past.