Update...
Well, they had the car for two weeks!
Engine need cleaning first so they could spot any spray from a leak - but valet guy away for four days. Finally got done but no external leaks at all. Head then came off and they found evidence of leakage between the cooling matrix and cylinder 5, very slight corrosion, but no cracks in the gasket. Head sent off for skimming but found to be perfectly flat apart from the area of the leak. Mega-light skim + pressure testing = good as new, no hairline cracks, no porosity, no nothing. Head refitted VERY carefully.
Then the fun started. Engine wouldn't start - spark, fuel, all OK. Diagnostics revealed nothing wrong. Anyway, a couple more days jiggery pokery later on the engine management system and distibutor and it's ready to roll, apparently. Got there again - engine wouldn't start. Another two days go by. Ready now, they said - and it was this time. Turned out to be overenthusiatic use of a pressure washer flooding the spark plugs.
The whole job cost the expected arm, leg, first born child and soul but not as much as expected. The story was that when the head was skimmed the first time ( because of cracked gasket), the garage used a sub-contractor, who then went out of business a year later. Suspect workmanship. Therefore, the garage conceded that the original job may not have been up to the usual standard and agreed to charge me labour rates as they were 2 1/2 years ago - £25 an hour less than now.
So although my wallet is in intensive care, it will survive.
This Lexus is a mean mistress...