ok, ive just flushed the roots and media in a strong oxy plus solution. Rinsed them in fresh water and left them to sit while i went to the hydro shop.
On my return, i stripped down the system, disinfected it with oxy plus, rinsed and setup again.

i refilled the res with half strength nutes, and a few ml's of PYTHOFF anti pythium additive per 10 litres...PH'd to 5.9.

I have kept the res level high, touching the bottom of the pots, as the guy in the hydro shop suggested that this was neccessary for the PYTHOFF to get to all of the root ball.

Kept the lights 12/12.

I realise i will have to keep adding the pythoff throughout the plants life as it kills the good bacteria as well as the bad, so without it, the problem would probably return.

I shall give it a week and see what happens.

Will let you know.