Thank you all for the suggestions and linked images. These were the very same images I was looking at (I have Collected Visions, but not HH boardgame - google images helped) as a starting reference point.
The problem with the HH boardgame version is that it appears to simply be a variant of the Repulsive class, whcih is already smaller than both the Desolater and Despoiler - hardly fitting for Horus's flagship/a Glorina class ship that can carry Legion style forces. Therefore, an upgrade beyond this is necessary. The other difficulty is getting a-hold of a Repulsive hull to use for conversions. I don't even have a stock Repulsive in my collection, let alone one for conversions!

(damn you GW...)
So, an update. I have now sourced an Emperor, a Retribution and a Despoiler as the basis of the conversions. From these, I am hoping for a Venerable Battlebarge (comparable to Despoiler class) for my Blood Angels and the Vengeful Spirit.
Venerable Battlebarge (Blood Angels):
Having test fitted some options and components, I feel the original plan of a Despoiler prow on an Emperor class is not working. I can't get the image of a spanner out of my mind...

So, an alternative option is needed. A stock Despoiler, converted to have side armour (as the modern BB) and Imperial weaponry is looking good. This does mean the Despoiler components wouldn't be available for the Vengeful Spirit though...
Vengeful Spirit (Sons of Horus/Black Legion):
Now, this has been the hardest to reconcile. The previous plan was a Retribution with additional Chaos Battleship upper hull at the rear, in the style of the Grand Cruisers. As mocked up here:

However, this may not be possible and doesn't really fir with the consensus I'm getting from the forum.
Having tried a few options and components though, a new plan is forming, particularly taking inspiration from the ongoing Heresy Era Battlebarge threads and this image:

So, what does everyone think of this double-decker version -

Too much? I'm worried it might be...
I am not sure whether to go for this, obviously with some fairly extensive work to get the prow right, or go for a version using the Chaos Battleship upper hull and try(!) and source a Repulsive prow and/or hull for the conversion. Decisions decisions! Any help or opinions are of course welcome!
Thanks,
H