UPDATE I have finally found what the problem was. When developing colour prints I use a Sodium Safelight with an opaque filter over the bulb (Kaiser Duka) The light from this is 'safe' up to a point with RA4 paper. The light output is variable with an internal baffle set at 50% and this baffle had come loose and I was getting the 'safe light' at full blast which was enough to give unusual colour casts.

I Repaired the baffle and last night and made 5 x 10x8 prints straight off with no problems at all.