Thanks all great advice as usual. Bought 500g of caustic soda (£2 couldn't believe it) added the whole lot to around 40l of cold water (instructions said to use cold) and soaked them for a couple of hours. Water is now black and most of the grease is off the rails. They now run back in fully but not out fully so some progress. Cleanest they have looked in years.

Plan to get some more later and give them another soak to try and get the final runner working a d remaining crud off.

Thanks again