date of this photo and anything on the purpose of the gathering

[arthurk]

I could be wrong with this (I'm better at places and buildings than people), but I think there may be a family resemblance between the three seated in the middle. The 3rd (and possibly 5th?) from the left in the back row look fairly similar too. Some of the others look as though they might be related as well, but to each other rather than necessarily the three in the front row. This makes me wonder if it could be people working in a family firm.

Besides that, I have a couple of thoughts about your great grandfather (the one with the sign). First, he's wearing a medal, though I can't make out what it is. Second, he appears to be raised up - do you know if he had any war wounds? What I'm wondering is if he might be in, if not on, some kind of wheelchair, and the 'sign' might be a board hiding evidence of amputation or other injury.

[Rhododendron]

I would assume your Grandfather is in a "privileged" position in that photo.  i.e. as anew club member etc. 
On closer inspection, is that something displayed on his lapel (the right hand one as you look at the photo)?

[jim1]

As ArthurK said, it's a medal.

[Gadget]

As he was a butcher, I think it's most likely to be some kind of club/association rather than work related.

[Mhillbilly]

thanks for all the thoughts so some more information may help
my great grandfather was never in the the military and date wise this is definitely from about 1900 to 1913. In 1909 he was 30.
Born in Manchester but moved to SW London when mother remarried, he was only (surviving?) child as  father died between 1884 and when mother re married in 1891. He was between 5 and 10.
Family moved to SW London between 1891 and 1899. He married in 1899 to a Elizabeth Boots from Fulham.   He was a butcher all his working life.  Elizabeth died in 1913 and there were 7 surviving children all under 12.  Definitely nothing to do with WW1 and great grandfather was not disabled in anyway.
I have cropped from the original without my photoshop (as raw as I have)  just my great grandfather and there does appear to be a medal.
 
 

[Mhillbilly]

more information
in 1901 census my great grandad was living in Clapham and was listed as a butcher at the meat market, not sure if that means Smithfield or was there one closer to Clapham?
he married into the Boots family who lived on Fulham Broadway known as Walham Green in that period.  At that time at Vanston Place just off Fulham Broadway marked on my 1892-1914 map is a Butchers Charitable Institution 'facility' looks like Alms Houses, my original thought was it could have been something to do with that but researching from the southern hemisphere I did not find anything.

[Mike in Cumbria]

The medal appears to be star-shaped if that helps at all.

[IgorStrav]

Is it my imagination, or does he appear to have a strap over his left shoulder?

I know there is a dark line showing the edge of the body of the jacket on both sides, but it seems to me that the strap is making the shoulder of the jacket 'puff up' around it on one side?

Though even if anyone agrees with me, I don't know what that would signify - a camera strap over his shoulder (were they that small then?), binoculars?

[Michael J]

Could the board be a 'thermometer-style' fund raising board and that the photo is celebrating reaching the total?

Michael.