All, as an update on this issue since May 2016
I ended up going through a lengthy internal complaints/dispute process with KBC on this issue for a good 6 months in 2016. The agreed outcome in the end was I could redraw the regular over-payments and lump sum payments made in 2015, but was not permitted to redraw any further over-payments made from 1st January 2016. This agreement suited me, as I had made considerable over-payments in 2015, and gave me access to a reasonable emergency reserve fund. I should add I also made further over-payments in 2016 and 2017, but these are not available for redraw. This agreement was also formally included in a FSO mediation resolution agreement on another issue in 2017, so I am pretty confident there is no chance it will not be honoured.
I am not sure why I was given the concession by KBC. Initially the only option available to me was to redraw all of the overpaid funds immediately. My argument was that I had received a letter from them confirming I could overpay the mortgage and redraw the funds at a later point. I had then checked this with the mortgage team and they confirmed I had this facility available to me - in an email. I then made a clear decision to liquidate other investments and paid a considerable lump sum against the mortgage - based on the facts they had given me in writing. I had also said that if the issue had not been resolved satisfactorily I would have brought it to the FSO for resolution.
I recently moved to a 10 year fixed rate with KBC, and I was advised at the time that the redraw is not affected by this and I simply pay 2.95% on any increased mortgage balance post redraw.
So the only advice I have for anyone who feels they may be in the "grey territory" here is to confirm what written confirmation you have around the redraw facility, what validation you done to ensure you were eligible etc. I would be suspicious that you will have any success in disputing this unless you validated with KBC that you were eligible. If you wish to PM me on anything further I am happy to discuss.