Right, well, none of your ideas worked, but thanks for the advice anyway! I'll explain why... and then I'll explain how I won, if anyones at all interested (my missus wasn't at all for some reason).
I just started churning out constructors, building starbases and maximising the cultural upgrades for hours, but because they had so many of their own, and because their population was re-producing like ants, it really wasn't making any difference.
So, I went back to a previous save, and instead of building any military I just left the few overlords and dozens of Avatars and Rangers in orbits, and then broke the Alliance I had with the Alts. After waiting for a while, I realised that although our relationship had deteriorated to Hostile, they just wouldn't start on me. So, having 3 Terror Stars handy, I positioned them near their their largest and best systems (making sure two of them were placed by the systems they had within 'my' territory) and then just blasted them. I took out the four huge systems they had withing my space, and also their capital in theirs. Obviously next tuen they took out a couple of my Terror stars, but I'd done what I wanted to, and so just sat back and waited. I moved the Military slider down to 10% - just to keep them moving, but pushed my Research and Social sliders to 45% each and kept hitting the enter key (for some decades actually). During this time their overlords attacked and attacked and eventually wiped out every ship I had. Because my populations on all planets was high, and because I'd kept building Social as well, I had Omega Defense, Star System Defense etc on every planet, so their Combat Transports which attacked every turn kept failing. This went on for ages but because they were spending so much on Military I soon went far ahead Technology-wise. Eventually (after about 50 years!) I managed to research Excalibur Tech. Then I wacked Military slider up to 100%, and made every planet build Excalibur, and I went from 0 strength Military up to about 225 in just a few turns.
I noticed that one of the minors had about 50,000bc in the bank, so I started on them and then offered peace in exchange for 50,000bc for 50 turns, and then I whacked my spend rate up to 100% as well. Soon my Excaliburs were wiping the floor with the Altarian Overlords - I didn't even lose 1 ship, and wiped out hundreds of theirs.
My Excaliburs were all around 735HP, 324 Attack and 66 Defence. Eventually, when they had no ships or Starbases left (yes I was laughing out loud by now - REVENGE!) I offered peace in exchange for a few of their planets and all their money for 50 turns. Obviously they were stuffed by now and over the next couple of years I just attacked and then offered peace for a few of their planets again.
Within the space of about 2 hours play, I went from having just 9% influence and having a military might of 0, with the Altarians with 90% influence and a military score of about 400, to winning the game.
Man I enjoyed that feeling. So, now to Crippling...
BTW at the height of my wealth and power, I was pumping 20,000bc per month into de-stabilising the Altarians and it had NO EFFECT WHATSOEVER!