Just had the same thing with mine, Mayo in breathers under cap and in dipstick etc.
Comp check was perfect and i used a sniffer on the coolant so no H/G fault.
Engine draws in air as it cools lots of journeys = lots of air, lots of moist air = lots of water evaporating as the oil gets warm this condenses and builds up in the pipes etc.
do a compression test, Use a Sniffer on the coolant or drain the oil out and check for water.
You can do all of the above yourself or for a reasonable fee a good garage will do it.
Better to be sure your £400 is well spent (£400 seems about £100/150 overpriced to me

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