I'd never heard of Sandbaker before but when googling it the first topic was your enquiry on another forum in which you give your great grandfather's name and a few other details.
The casualty roll confirms 3027 Sgt. J. Mellett 2/Northants (MI) severely wounded at Sandbaker 3/7/1901. He is also listed as wounded at Enslin 7/12/1899 but with the name Mellitt. This was presumably a relatively minor wound as he continued to serve.
Now for the good news. He was what is known as a 'PIN man'. These were men who were invalided to pension as a result of wounds, injury or sickness and are a particular interest of mine because of the wealth of research material available on them. Their files are in the series PIN/71 at the National Archives and they frequently run to well over 100 pages, sometimes over 200 pages. Much of this relates to pension awards but typically they also include correspondence to and from the man, medical reports (their disabilities were reassessed over a long period), discharge documents, defaulters' sheets and death certificate.
These file headings give only the full name of the man and the years of enlistment and death so with common names you can't always be certain it's the right man. However, Mellett is an uncommon name and file 71/4263 is that of James Mellett 1891-1948.
The only practical way to retrieve all the available information is on a CD-ROM which my researcher compiles with a digital camera. If you'd like him to do this for you let me know and I'll PM his details. He can also check his medal and clasp entitlement on the medal rolls at the same time. It's also possible James's attestation and service papers survive in series WO 97 but even if not the PIN file will contain much more information than that.
David