We are also trying to get hold of similar records and have been a little frustrated with the system on the archives. Having found the one we are looking for, we are now having problems ordering it!
Anyway since I've worked out how to use the archives to find records, I have managed to find the location of the record you are looking for and the reference number so I hope this helps.
It is here:http://www.nationalarchives.gov.uk/catalogue/displaycatalogu...
Reference no: AIR 79/609/66172
This was the only record I could find for an Alexander Kendall so I assume its the correct one.
His service number is 66172 and according to the 1918 muster roll, he enlisted on 13/3/1917.