Hi Martin,
I bet it has happened more often than people think. Especially now that so many people are taking lovely images of galaxies like yours and what an absolutely superb image it is - congratulations. Fact is there are a lot of stars in that image and unless you are undertaking a deliberate search with prior reference images then you have got very little chance of noticing a new star. 2007sr was 12.9 magnitude when discovered on 18 December 2007, even at that bright magnitude at discovery date with so many stars in the field most would be at a loss without checking against previous images or the DSS images. So maybe this is the wakeup call for all you galaxy imagers - if you are going to take beautiful images of galaxies as per Strongman Mikes wonderful recent Grus Quartet then why not go that little bit further - you might as well check them against the DSS images, you may well get lucky.
Hey it even happened to a well known Aussie Scotsman with the LMC, he even thought something looked "odd" but didn't check his images until later and the rest is well, history... congratulations to Shelton.
I missed a mag 17 SN in July just a week or so after my discovery in NGC5967, if I employed image subtraction I would have got it, I don't, I just eyebeall each galaxy and I was tired, so I say that is a "bugger".
Bojan is near the money - it is even more important to find you have a pre discovery image of a Supernova in your image, or even no Supernova in an image say a week prior to a discovery being made. This will add value to further research of the SN and needs to be reported to the IAU per IAU procedure. So it is important to keep the date and time (in decimal time) of any galaxy images you take - it could turn out to be very valuable information.
Either way, I say what a fantastic image you have Martin. I would suggest you send it to Dave Bishop for inclusion in his images of Supernova on the Bright Supernova Page
dbishop@vhdl.org - there are not many colour images there and yours is a corker.
PeterM.
