Hello Elaine, welcome aboard. It would be helpful if you could post the details of the regiments and the medals earned by your grandfather. Also, date and place of birth would be helpful.
The MC was instituted on 31 December 1914 so not from the Anglo Boer War period. It was a gallantry award for junior army officers and warrant officers. You'll be able to find the announcement in the London Gazette which is searchable online. Sometimes the citation is included with the announcement and other times it isn't and just the award announcement is listed.
You should also be able to find the listing of his commission and likely promotions. The LG can be difficult to search and often seems to spit out "no record" when presumably the load of traffic is great for its servers. If you can't find anything on your first attempt go back and search at other times. I always start with a known reference and if that comes up as "no record" in a search I know that the LG is having issues again and try search again later. Early hours of the morning seem to be my most productive time when searching the LG.
Presumably your grandfather served during both the Anglo Boer War and WWI. As an officer there is a very good chance that his service records survive. You should be able to access his records in either WO 339 or WO 374 with the index in WO 338 at The National Archives. If he stayed in the army after about 1920 his service records are likely still with the Ministry of Defence and they can be contacted for the records.
The practice of writing a daily unit war diary started after the ABW so it can be difficult to trace exactly what an individual did during the ABW. Mostly, regimental/unit histories can be helpful in piecing together what the unit was doing. As an officer there is a good chance he would be mentioned from time to time. For WWI you'll be able to consult unit war diaries which may help you.
Cheers,
Mark
