Thanks for your replies, the camera has an IR anti-bleed filter, details of camera at www dot tradecctv dot net /specs/TC310HQ1.pdf , so i don't think this is the issue. I've also tried cleaning the lens tonight with no success.
Any thoughts why the camera is not switching to B&W despite the IR being on? Surely if the darkness is enough to trigger IR it would trigger B&W also?
I noticed last night while the street light outside to the left of the picture warmed up and was not at full brightness that the IR was on and the image was B&W and much much clearer. Once the street light was at full brightness the camera switched back to colour and the image returns to not being very clear.