The man on the right is definitely my Great-Great-Great-Grandfather and he was born in 1860, so if he is about 50 in the photo it would put the picture around 1910 which I think seems realistic. The first one, who is still unknown, came out of a photo album which was compiled in 1924 and all of the photos in it were taken between 1918 and 1924 (at least the annotated ones were). If he is, as you say, in his 30s, that would put the date of birth for this man in 1894. Interestingly, my Great-Great-Great-Grandfather did have a son born in 1894 called Heth Jones who we don't have any photographs of, so it seems highly likely that this could be his son Heth... Heth died in 1940 when he was hit by a motorcycle during a blackout and I later found out that he was discharged from the military because he was partially sighted (perhaps also a factor as to why he didn't see the motorcyclist coming). There might still be people on his side of the family who still remember him if that's the case, although I'm yet to get in contact with them. It might be "clutching at straws" but now I have a better idea of who it could be, I at least know where to start looking for an answer. There were also ten children in this generation of the family (i.e. my Great-Great-Great-Grandfathers children) and there were pictures of at least six of them in this album, so it would make sense that there is one of him as well.
Thanks for all the help on this thread!
Henning