it's often a dilute blend of lager and line cleaner
No, it's really not.
I used to be a licensee and the process is :-
1. Undo all the grundy heads and attach to the line.
2. Fill up the right mix of line cleaner and water into the huge bottle
3. Draw the liquid through the beer taps until you feel the mixture of cleaner and water - it should feel soapy.
4. Leave for 30 mins
5. Empty the huge bottle of the line clean mix - it makes a great cleaner for cellar floors.
6. Connect hose up to the water line.
7. Draw the liquid through the beer taps until it doesn't feel soapy.
8. Reconnect the grundy head to the keg
9. Draw through the water until it magically turns to beer.
A good landlord knows how long his lines are and how much cleaning solution they take. If you get diluted beer that tastes of line cleaner then the landlord is a feckless tool.Find another pub.