I don't know if this will be of help to others but its worth putting here in case others experience similar.
I've had issues with my beer slowly souring in the keg, my dispensing set up from a sankey keg was a Pegas Ecojet counter pressure bottle filler which has a connection for a Perlick type tap, which I bought a chinese copy from Ali Express.
The whole set up was flushed with line cleaner regularly and stripping down the tap and cleaning it didn't rectify matters. So I've swapped those out in favour of a Dalex tap and I'm experiencing no issues now; it would seem that somewhere within that setup infection was able to pass back through the pipes into the keg.
I suspect strongly that its the tap that was the issue rather the Pegas filler as adding a cap to the end of the tap slowed the souring. I shall use the Pegas as and when I want to bottle some, but am pleased that swapping out the tap has worked.