Following on from the post above as said there was no list of 'reserved occupations' as such but under the terms of the Act (http://www.1914-1918.net/msa1916.html
) a man could appeal to his local tribunal for exemption on the grounds that if he was called up it would cause hardship or the failure of his or his employer's business. I recall, for example, seeing an appeal by a painter and decorator. Other documented examples include grooms and hunt servants. The Tribunals were usually formed from the usual suspects of local worthies and as ever it seems although they were generally unsympathetic if you had the right connections it was possible to gain exemption on the most tenuous of grounds
An exemption granted by the tribunal was time limited, usually to six months, and if a man's circumstances changed then he would become liable for call up.
As for the exemption of agricultural labourers there was a debate in Parliament in 1917 where, when discussing an extension to the Act, an MP describes a certificate of exemption issued by the Board of Agriculture and goes on to say the Board and the War Office were at 'daggers drawn over this issue' (of exemption).
The debate goes on to discuss the real dilemma over food production versus military service and whether or not agricultural workers should be automatically exempt.
There were similar debates about mine workers who volunteered in such numbers in 1914 there were real problems. They provide a good example of later exemptions so a miner would be exempt as long as he worked in the mines, if he left then the grounds for exemption would cease and he be automatically liable for conscription. The phrase 'reserved occupation' seems to have crept into these debates but I don't know where it originated.
The threads referred to above describe a few rare examples of where records have survived but the best place to look is the newspaper archives who invariably published the decisions of the Local Tribunals. Juxtaposed against the casualty lists published in the same editions, whether intentional or not, it was another subtle pressure to encourage men to enlist.