"Civil service regard software as computers which is admin/secretarial. to them. And offer 20k. Max."
The mid level project management types that do SIAM work are actually alright in this regard. They're likely to map into the grade 7/6 range, so could easily be earning in the 50-60k range. Probably a slight knock down from what they'd expect on the open market, but not hugely so.
The problem with the reverse-TUPE into the civil service lark is you're locked into your terms forevermore. The only way you can ever progress is by switching fully into the CS grading structure. More to the point you're stuck in the ludicrous CS ways of working, where by definition a 6/7 is a team leader, whether or not they want a team to lead.
It's a culture shock.