No worries about telling me things I may not want to hear. I am well aware that there may be things that I haven't considered, which is why I asked. I would far rather be told now that I am being naive, then discover it for myself after getting the dog. 'Tis why I asked 
When I said an hour a day, I meant in one block, one to one time with just the dog, certainly to begin with at least. I can easily fit in 2 half hour walks or 3 20 minute ones, but I would prefer, for the dogs sake, an hour long block so that the dog can have a good run out. The nature of my DP's work means that there isn't a definite time he is home by so that coupled with general family life means I can't guarantee that every single day, I will have a minimum of an hour to take the dog out alone. Obviously when the dog is trained a bit I can take it with me when I take DS3 out for a walk so I'm not sure that that means the dog wouldn't get enough exercise, but I am willing to be proven wrong. Again, it is why I asked. You lot seem to have way more dog owning experience than me so I am shamelessly using you as a font of wisdom.
Scuttlebutter sorry, in my (vain) attempt to be brief I obviously left out some important details. It wasn't my choice to leave my dog behind before. Given the chance I would have brought her with me. However, as I was fleeing domestic violence (aimed at me, not the kids or dog) and it was as I said, a midnight flit, I left in my mother's tiny car and had the choice between bringing the dog or bringing the kids. We were going to be staying at my mother's for a then unknown length of time as well, as it wouldn't be possible to have the dog there, it simply wouldn't have been fair to the dog to have been in a 2 bedroom house filled with clutter and no back garden to speak of, especially when I was at a time when I couldn't walk her myself (broken leg) and my mother dislikes dogs so wouldn't have taken her.
I did try and get her rehomed after I left, but it was difficult getting sense from anyone or finding anyone able to help since I was no longer classed as her owner.
I couldn't think of the best way to phrase what I meant, so went with 'pecking order', but I guess what I mean is that it would make sense for a dog to be out of sorts when a new addition arrives, whereas if DS3 were already here, the dog would be coming in to an established unit so there wouldn't be the issue of fitting in, then fitting in again IYSWIM?
Yes DP had dogs (anything up to 4 at a time I believe) as a child and it is only his parent's ill health that stopped them getting more as the others died of old age, so they no longer have any. He doesn't have any himself as he sometimes works long hours and is often away for the weekend visiting me, so it wouldn't be fair on the dog. Obviously most of that won't be an issue once I live there as well. He is keen to have a family dog and TBH it would be more for me and him than for the kids. The kids have always had it explained to them that we couldn't have one before as it wouldn't be fair to the dog, and that there are lots of things to consider before getting one, so them waiting isn't an issue at all, as I agree with you that it certainly isn't cruel to them, and may in fact be doing them wrong as it would teach them that animals needs don't need to be respected which wouldn't exactly be a good lesson.
epiphany that is a good point and I was making an assumption based on leg length and limited observation of the big (boisterous) dogs I know and the small (laid back) ones. I should have realised that like children, Every Dog is Different.
FWIW, thank you all for your replies and I am taking them on board. As I have said, I do not intend to get a dog unless I am certain that I can offer it the care and attention that it needs, and in any case am not considering it for a year yet at least. If it turns out that our situation isn't appropriate for having a dog, then I will of course be upset as I do love furry hugs [winks] but I would far rather get as much advice now as possible and decide that it wouldn't work for us, than just blindly do it and end up doing wrong by the dog.
Gah I do hope I'm not coming across as an idiotic 'zomgz i really want a puppyyyyyyyyyyy they r soooo cutez!' type and that I am making it clear that I am trying to give this as much consideration as possible.