Could anyone advise on likely cause of soft brake pedal with engine running? Pedal soft and barely does anything for top 2/3rds then bottom 1/3rd travel feels normal and brakes ok. With engine off brakes initially soft then hard after pumping, with engine on not actually driving can almost pump up but pedal sinks under pressure and cant seem to build up much hardness while underway. Have changed pads and had a brief look at hoses while doing so which seemed fine, not losing any hydraulic fluid so had ruled out master cylinder or a leak somewhere. Could an internal master cylinder failure or some kind of servo issue cause this? Thanks in advance.

Welcome!
IMHO:
Engine running and pedal moves further is normal due to servo being charged up (3 or 4 pushes should exhaust that assistance.)
If pedal is repeatedly soft/spongy but you can pump it up somewhat, and it stays hard and works, the first thing is air in the system.
If it's spongy and pulls to one side or one or more brakes don't work at the wheel, it's something seized causing sprnginess
If pressing the pedal hard, maintaining pressure and the pedal slowly sinks, it is an internal master cylinder leak that needs looking into.
Has the system been bled including the "service bleed" function to flush air out of the ABS system?
What is the history and what work has been done?
Always ready to hear other ideas on a long distance diagnosis with limited data!!!

Thanks for the info, sounds like the issue is likely to be master cylinder then as pumping doesn't make a difference and also with how suddenly it changed I can't think how air would have got into the system so quickly. Only other work that I think could be relevant to brakes/hydraulic system was bleeding the clutch a couple of months ago, car is 16 years old and has gone through quite a few owners so don't know full history but think in general there's been little maintenance.

Brakes haven't been bled before but I'm thinking it may be worth doing before trying anything else for cheapness. I don't have a device that can control ABS valves, is bleeding without this and only going round callipers worthwhile or is it all or nothing? I imagine the system would need bled after changing master cylinder anyway so if software to control ABS valves is needed it might end up going to a garage.

We (me and MBH) have changed brake fluid regularly using a simple bleed tube and never had any problems BUT never had any air in the system so ABS likely not involved. If no real history perhaps worth a try at minimal cost
I believe Forscan has an ABS Service Bleed function but as said, never needed it, that would be the next level.
After that it needs a definite diagnosis to ensure the correct component is changed.
If slow decay of pedal with sustained pressure, master cylinder is likely but don't do it without much more certainty than taking advice from a random bloke on a forum!

Let us know the outcome.

Took me a while but seems to have improved itself, thanks for your help and suggestions for what could have gone wrong. I'd be totally guessing at what could be the issue but suspect since only pads have been changed it was something sticking somewhere which has freed itself up. Attempted bleeding but was a very poor effort as the nipples look and feel like they'd snap if turned. As luck would have it the car's MOT is due soon so will hopefully reveal if a component has failed, will update if the MOT reveals anything.