Ahh flimpo I totally undertand your situation.
As you know from previous posts I spend most of my time alone with Emilia as my OH works long hours and is often not home until after midnight. I know how it feels to be chief cook and bottle washer, and although the babies are sleeping all night it is still exhausting, especially when they are having a rough day. It def effects your relationship as well, its so hard to reach out to the person you love the most, when your lives are so different. Plus I know Emilia takes all my energy, often I dont have time to deal with OH's problems.
If you cant go away for OH birthday, could you perhaps organise a special meal at home. Then you could perhaps have a chat with OH. Maybe even get your mum to have Cam for the night, so she gets her grandma time? My MIL lives next do so I feel you pain on that score

I wish I were back in the UK already, so we could all meet up! I would love to go back to the leeds area

Depression creeps up on you so quickly and you are so brave for recognising that it might be appearing again. Defo speak to your HV or a good friend. I know from experience it is awful, and so uncontrollable.
As you say chin up, and have lots of hugs with Cam
xx