Thanks for this extremely insightful post Ent319. The information out there on the SPSPS is extremely limited, probably in time as the years go on this will change, but for now we're limited.
I think what you suggested regarding contributing the difference in ASC between pre and post to an AVC/PRSA is a really interesting concept. I joined the civil service 2 years ago as an AP at the age of 27. I'm lucky enough at this age to be a homeowner, with a manageable mortgage so this is something I'm really considering. I might not max might my contributions, but definitely happy to contribute something.
So based on the above, and I've heard similar elsewhere about Cornmarket. I think one of the better options is to setup an AVC PRSA through Davy Select and invest in something like a Vanguard Global Stock Fund. (0.75% AMC, 100% allocation, no fee).