Mothercare are pretty good. We had a pram that wasn't purchased through them that developed a fault. We couldn't get hold of the original retailer but Mothercare ordered the part for us. Obviously we paid as they weren't the retailer we purchased it through
I believe that they may also offer loan prams while yours is being repaired.
You'll need the receipt if you want them to honour a warranty and probably to get a loaner buggy too. If you are willing to pay for the part, they'll order it for you without any proof of purchase. See my previous posting.