Hedgemonkey, have a full bacteriological breakdown done. It will show up where the problem is by the type(s) of bacteria in the sample. Your dairy should already have suggested this, ask them where they get samples tested for Bactoscan, any of the labs with a bactoscan machine will be able to do a full breakdown for around £40 and advise you on action to take.
Be careful the labs can have a £100 invoice minimum, get the dairy to pay for it and knock it off your milk cheque.
There are 3 labs I know of, ADAS - wolverhampton, whatever Genus is called now - East of York and Global at Mirfield, West Yorks (01924 499776).
Good luck