Bit of a weird one this, had to do quite a bit of work on my D2 to get it through it's MOT (steering knuckle ball joints, front shock bushes, rear pads (ended up needing new calipers, carriers and discs))
Having done all of that thought I'd be ready for a retest but the brake pedal is now soft with the engine running. With the engine off it's nice and firm and holds with constant pressure, switch the engine on however and the pedal goes straight to the floor. Pumping the pedal gives pressure but if I hold my foot on the pedal it slowly drops to the floor. My thoughts are servo problems, the brakes have been beld a couple of times so it's not a problem with air in the system, everything was fine before the brake change so it seems odd that it would fail while not being used.

Any thoughts/ideas/recommendations?

I guess not :/

Put this question in the Discovery section,you will get more response in there.

did you disconnect your calliper from the brake lines when you did your swivel ball? if so, have you bled your brakes?

If the braking system is anything like the Defender, there's an awful lot of air can hide in there. I usually have to let it settle for as while and then bleed again to get it all out. Yes, it will feel softer with the engine running because the servo is helping you. Let it rest overnight and then bleed some more fluid through the following day and see if it feels any better.