In the absence of a reply I'll assume you have resolved this problem.
For others in the same predicament firstly check if it is the strap which has failed or corrosion of the fastening to chassis. A new fastener bolt may be all that is necessary.
If you do require a strap, identify which one as I believe the two are a different shape.
Next, think about which vehicle is like the Captiva.... the answer is the Vauxhall Antara. (Same mechanically pretty much with just a different body shape).
So try the Vauxhall service agent near you. Be mindful that Vauxhall has changed allegiance and some stubbornly will now no longer service Vauxhall parts and others still will.
Another option is to try the internet ie eBay, Amazon, and Motor factors or the Breakers Yards.
Yet another solution could be to fabricate your own. Galvanised construction industry 'banding' is available from places like Tool Station (50mm width). When removing the old strap try not to bend it. Keep it as a template for your new banding. It is much easier to bend to shape on the garage floor or work bench than it is laying on your back trying to bend shape upside down under the vehicle.
If the rubber anti-squeak / anti-rattle strip has perished buy a new length off eBay or your local motor factor.
Plenty of possible solutions to this one. Just think it through.