If he was at the seige of Peking, you should be able to find his name on a list somewhere. There were only 3 officers and 79 enlisted men defending the Legation Quarter.
Seymours Expedition was a relief column intended for Peking. Admiral Sir Edward Seymour left Taku with a contingent of Royal marines and Bluejackets and assembled a multinational force at Tienstin. There were about 2100 men, 915 were British. Seymour was soon surrounded - but was releived by a follow up column of 5,000 allied troops.
At any rate, I believe the troops at the legation were from the Royal Welch Fusliers, rather than bootnecks. The British also moved units of the Indian Army into China during the war.