Right! I'm back! (Time to put on my big-girl pants and get on with it.)
So my main issue was that the sw really did not use child-friendly language with my dss and he found a lot of the meeting inaccessible/daunting for that reason.
Also, don't bullshit me and tell me it isn't a test, the giveaway is the word 'assessment' surely? 
I haven't really been pulled up on my life decisions -yet- BUT I did not realise that the report would be an assessment in such clinical terms. i.e. dss has lived with us for 4+ years with zero maintenance from his birth mum and he clearly wants for nothing and yet I need to disclose my salary to prove that I can 'provide for a child long-term'. Huh? I earn more than dh so have actually provided him with more than his biological parents put together but I must show this on paper.
Finally, the smiling, polite healthy-looking boy sitting with us in the garden was not proof enough of my 'parenting capacity'- highly detailed references from referees who can vouch that I understand that what I'm doing is a lifelong commitment will need to speak up for me and I will need give lots of examples proving that I am a fit parent. So I will have to explain that I was dss' school uniform, buy him nice Christmas presents, take him out for his birthday etc... to be blunt, I thanked my lucky stars continually throughout the interview that I earn a good wage. Something told me that if not, I wouldn't have stood a chance and I really don't think that that's right...
It is a patronising, draining and in many ways draconian process that I would only recommend to step parents who truly, truly have a deep, heartfelt and enduring bond with their stepchild(ren). I faltered very slightly on the Tuesday evening to be perfectly honest but one look at my darling boy's face tells me that I can't give up now, even though I know that there is far, far worse to come.
He deserves to have a mum and I am lucky to have him. ❤