replace battery (code S920SW) D&G Mens Sandpiper watch white face

my wife wears a Gents D&G watch which just recently stopped working (dead battery).
The estimated costs from 2 places I visited were £10 up to £25, the biggest issue being that 'it cost more so we can certify its waterproof again' , well call me tight,but Im not spending 10% or more of the watches original value on a battery replacement. Daylight robbery, forcing me to go down the route of changing it myself.

heres the watch in question
D&G Mens Sandpiper watch silver strap with white face

I would thoroughly recommend buying the proper tool to remove and replace the watch back.
The 2 choices appear to be

An adjustable 2 pronged tool
MKCLOCKS Pocket Sized Waterproof Screw back Watch Case Opener (ebay)
or
A 'jaxa' tool which is 3 arms on a handle

I bought the former as it was cheaper (£4.99)

An the most important item, the battery code ....
1.55v Silver Oxide Watch battery(SR920SW)
plenty to choose from on ebay. around a £1

voila job done for £6 [UPDATE : Nov 2013 : nearly 3 years from the post above the battery finally stopped, and ireplaced with exactly the same again. top quality]