Please, can you let me know where to buy a replacement bumper bar for the Mothercare Xtreme pushchair?
I would love to tell you but spare parts are not just available to buy in a shop or so. These are usually only ordered based on a need, and this does the distributor/the retail (shop) you ought it at originally, usually.
Do contact the retailer you bought it at, firstly, asking for the part you need even if it would take a fee to get it. As a discontinued Mothercare model, however, it might prove undoable for the retailer.
Next, I'd contact Mothercare/Boots customer service and ask for help... They, however, say this on their webpage: "We don't have a list of specific spare parts available. However, your local store will be able to advise if the item you need is available and they can order it for you."