After more staring at track plans, measuring baseboards, etc. I'm definitely of the opinion that 3 goods yard sidings is too many for the space I have. What I really don't want to happen is to end up with just an unbroken swathe of tracks between the front of the baseboard and the platform at the back, which would basically be the case if I filled the space with sidings.
To go back to the subject of the engine shed... I was flicking through Barry Norman's book 'Designing a Layout' last night and found myself looking at the Aldeburgh track plan again. OK, it's the opposite side of the country, but this is essentially the same as my previous track plan with the engine shed coming off the back siding - but instead of the siding being used for a loading dock or such like, it is simply an engine servicing road, which makes the idea of the engine shed adjoining it perfectly acceptable. I don't know why I didn't think of this earlier, because it makes perfect sense.
It's back to plan 7246543 then!!! As BR used to say - we're getting there!