I can confirm the date as it is stamped on the back of the photo, 1941, I have only just identified the lady.
I should give some explanation, sorry this is long. My aunt sent me the family photos to scan. Many of them were clearly incorrectly labeled, I suspect by my aunt.
We have a large family my great-aunt Sarah the oldest was born in 1877 and the youngest was born in 1900; most went to Canada or the US in the 1910-1920s This young lady was labeled with the married surname of my great-aunt Alice, but clearly this was wrong.
Also I received a number of letters from my grandfather and other family members to my grandmother and my grandfather's address book. In these they talk about Dorothy and her brother and one aunt, Annie, in particular in Canada writes about her in a very familial way, which I thought was fairly odd. On searching my grandfathers address book (as it is clear he met her from the letters) I got the surname Manbre. Knowing the family were in Liverpool at the time I found Dorothy Manbre with the mothers maiden name as the correct one for my family. The birth certificate clearly shows that my aunt Annie was the mother but she was using her 'husband's' name. However:
I can find no marriage certificate
I can find no divorce
Annie went to Canada in 1926, married there and is given as single on her marriage certificate
Dorothy and her brother were not mentioned in Annie's obituary
It is clear that Dorothy had not met my grandfather before the early 1940s
Before anyone asks if I am certain that this is the correct Annie, she had a very distinctive middle name that she always used.
So the lady I know was 21 at the time of the photo, on the back it says 'to Tom fondest love Dorothy' with the date 30 September 1941. Tom (my grandfather, he died in 1942) was in the navy, her brother I know less about, I'm not sure, but at this point he had been in India for 4.5 years and I've just managed to confirm that he was with the Scottish Borderer's.
I am at an early point in my research as it has taken me in a direction I had not expected. My aunt knew nothing about any of this. She was not married and appears not to have married at least until after 1946.
So in short, I could have messed up but I don't think I have as I've been very careful, but I had a thought, this is her Uncle John, my great-uncle, he fought in WWI in the RGASR (it is marked as Roderick, Roddy, but if this is an Royal Artillery cap then this John), which answers two questions at once. John died in 1937.