OK got to the bottom of this, nothing I read inclduing the link abve suggested delays like I was getting.
Went to use the flash for the first time since I had those issues this weekend, and found the batteries were flat, unusual, as I had turned it off.
I thought nothing of it until I noticed that I my recharge time was heaps better on the new set of batteries (same type as before 2300mah NiMh's), I assumed it was due to the flash not having to work as hard (bouncing it off a lower ceiling this time).
What confirmed that something was night right was when I went to recharge the old set of batteries, one of the cells was completley dead and would not take any charge, where as the others were fine.
A quick play around with the 420EX in simular conditions to those I was in a few weeks ago confirmed it, the dud battery was messing with things, not sure how, I wouldn't think the flash could run on three batteries alone.
Unfortunatly I didn't check the charge level on the three remaining good batteries before putting them on charge.