Seat part

Hello all. I've just fitted some seats I bought off eBay and the passenger one is wobbling in the rails. On inspection it appears to be missing a spacer of some sort - a white plastic cap off of the base of the frame. Does anyone know what this is called so I can find a replacement? Better still, does anyone have a spare they could flog me? I should have taken a photo but forgot..

Oh, it's a mk1 cab but I don't think that makes a difference.

Cheers

http://www.ebay.co.uk/itm/NEW-SET-SEAT-RAIL-GLIDES-GOLF-MK1-MK2-MK3-gti-CADDY-POLO-LUPO-AROSA-IBIZA-TOLEDO-/350583546240?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item51a0686180