TD5 spongy clutch pedal? Master or slave cylinder?

As per title, not long replaced the clutch kit in my TD5, clutch bled ok, but has felt spongy ever since. thought I'd give it a couple of weeks to see if it improved, but still the same. must be one of the cylinders on the way out, but master or slave? which is most likely?
Tempted to go with the slave, but it wondered if any of the guru's on here knew if TD5 master cylinders are known to fail often.

usually the slave cylinder gives up first...but they are not extremely expensive so the best thing is to replace them both cos putting just one of them new, the other one will cede in short time anyway cos the tired piston seals will be exposed to the bigger pressure of the new ones...

The clutch damper is also likely to give problems, but as it's part of the master cylinder, a new cylinder will include it.

Peter