think i solved it - went under the house (where the garage is) and found another clip and some o rings and rubber washers. we had a pest guy spray under the house and he may have kicked it onto the garage floor. seems to suggest that previous owners did some diy work, as there are a large number of nails, screws, odds and sods under the house. the other clip is larger and rusted but i guess the smaller one could have been under the house for a long time without rusting. are these circlips used for domestic work as well as cars?
below is a pic with all the other bits i found: