I am trying to understand my Grandfathers service record and I am having difficulty. Can anyone please help with the following:
Alfred George Hurn  Date of entry 23rd May 1909 Official Number JX143377 What branch was he?
In name of ship column on record, in some cases two names are listed one of which is in brackets. What does this mean? and what do numbers in "List and No column indicate?
I am sure that I will have many more questions so thank you in advance

If you look carefully on his service record, it will tell you what his rating was.

The service number is odd if he really joined in 1909 - the X part of his number only came into use after 1925, likewise the official number 143377 indicates a post 1925 Seaman branch entrant.

Why two ships, one in brackets? the first is the parent establishment, usually a shore base, but sometimes a depot ship, that was responsible for pay and admin. the name in brackets is the actual ship served on.

Without actually seeing a record it is difficult to answer the other queries.

Might the number J4889 make sense as this is written faintly beside the other number.He appears to have joined in July 09 as a Boy 2nd class and was then made Boy 1st class in March 1910 and then seaman(i think) in Feb 1911.

Yes, J4889 would fit; a new numbering system came into force from 1 January 1908, starting with J.1, so J.4889 for mid-1909 is about right.  He would have been seaman branch.

Have now moved on to later part of Grandfathers service and once again I am confused: He was pensioned in Feb 33 with rank of PO. He then came back in May 33 with rank PO Pens NCS? There is a gap then before he then appears to have gone to Drake as a PO Pens from March 34 to Dec 35 then on 1 Jan 36 his rank is shown as AB then on 2 Jan 36 his rank is PO again.This then remains until he was finally released in May 1947. Do you have any idea why his rank may have changed so quickly?
Finally do you know where I may find out where ships attached to China Station, Mediterranean Fleet etc actually were at any given time. Many thanks

 "..where may I find out where ships attached to China Station, Mediterranean Fleet etc actually were at any given time. Many thanks"

What ships & when - I may be able to give some help

The list is long I'm afraid but here goes:
HMS Leviathan Grand Fleet 1910
HMS Commonwealth 2nd Division Home Fleet 1910 (second half)
HMS Carnarvon 3rd Division Home Fleet 1910 - 1911
HMS Monmouth China Station 1912 - 1913
    "  "    Home Fleet 1913
HMS Marlborough  June 1914 - September 1915
HMS Nerissa  March 1917 - December 1919
HMS Raider 1919 - 1922
HMS Warwick 1923 - 1926 Mediterranean
HMS Erebus 1931
Sorry if I am asking too much but I do not know how else to find the information. Thanks