Is there a place in Moreton in Marsh where we can check our luggage for a few hours?
Try searching on either stasher or nannybag to see if there are any local businesses that will hold bag.
I believe in an earlier post you were looking for storage in order to visit Blenheim. You will probably have better luck finding a place to store your bag in Oxford. If you are coming by train between London and Moreton-in-Marsh, you will change trains in Oxford.
From Oxford, you can take a bus to Blenheim. I spent the better part of a day at Blenheim so I’m not sure I would choose to try to visit while en-route between London and the Cotswolds but YMMV.
They have lockers at Blenheim Palace-
You are permitted to take a small bag only into the Palace. For larger bags, we ask that you use our locker service. Lockers of different sizes are situated outside the Blenheim Palace shop. We charge depending on size. Pricing is £4.00 for a small locker, £6.00 for a medium, and £8.00 for a large locker for 4 hours of storage time. We charge an extra £2.00 per hour over the 4-hour period.
That way you can take the train to Oxford, bus S3 from the Station to Blenheim and then the S3 back to Oxford Station for your forward train to Moreton, without having to go into the city to find a luggage drop point.
How are you managing your bags on your walk- carrying them (backpacks) or using a Sherpa service?
Maybe e-mail Blenheim to check that their large lockers fit your backpack dimensions?
Are you staying in Chipping Campden (per your original post) or Moreton. If in CC there are buses from Moreton Station at 1725 and 1845.