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I have great respect for the setter here. It must have been incredibly difficult to get all of it to work thematically. I initially made the mistake with the cipher of putting zero at the start of 1,2,3.... instead of the end. Particularly impressive was the way clue 21dn was itself thematic!
As usual thanks to everyone for their help with this one. If anyone is still struggling can I confirm that the theme is nothing to do with space travel!

Unclued, I guess the setter worked backwards by constructing Grid B first and then imposing a symmetrical set of bars to produce Grid A. The challenge in working out the clues is to produce combinations that lead to a unique solution that can be obtained without too much brute computation, and I think that the balance between computation and logic was about right.

Murky, there is nothing significant about the positioning of the zeros. It's maybe wisest to ignore the 'tips' from that contributor.

I guessed two of the words that were supposed to appear in the grid but they would not appear until I messed around with the encoding and shifted one line one step. Of the 26 possibilities the right one is not the most logical, though it is the second most logical.

While I certainly admire the construction of the grids and the clues I'd have got rather more pleasure out of it if the entrance hall to the establishment hadn't been so long and tortuous.

Some elements in multiple clues seemed redundant (eg the second part of the equation in 23d), though they didn't seem to aid the solving as redundant clues sometimes do.

Glad you found the key and got to your suite Murky ! The exact order the clues are solved in probably determines which of the multiple clues contain redundant elements - there were maybe 2 or 3 instances of this for most solvers, though they were reassuring confirmations that one wasn't heading down a blind corridor !

Thanks wintonian for you help in kick-starting my attempt to solve this crossword. I see the logical steps you have taken and will attempt to proceed along those lines. Help much appreciated

Goshawk (and anyone else attempting this) - I don't know if you have access to Excel but if you do it could significantly help computations in grid A. I started coding manually which proved very laborious (and risky in terms of typos). I then realised that if the clue data was copied and pasted into Word it was very simple to then 'mass edit' the data into Excel format, by editing in an asterisk for multiplications and prefixing an '=SUM(' and suffixing the opposing bracket. The resulting edited data can then be pasted into Excel cells to produce perfect results (I used the top row to represent A1 through Z1).

That said having got through Grid A I still haven't the faintest clue what to do with Grid B, but that's another story.

I would have got off to a much better start if I'd realized earlier that in 1a, for example, both K+ILO and (AL+P)H give the answer. I thought it was just saying that they were equivalents.

S - in grid B look for consecutive number pairs in rows 2 to 5. Box them off and you will be left with a blank perimeter and blank central column. Hope that gets you started.

Thanks Unclued but I was at that point 24 hours ago and still none the wiser (even though I spotted the absence of an unlucky number)