Tony, as you have found out, this is a difficult problem to solve/cure. I have replaced virtually all of the caliper parts and the knocking remains. Having had everything checked twice by an independent garage and Lexus Bristol I'm convinced, as are Lexus Bristol now, that my problem lies with either the suspension top mount or the shock absorber (these are known for having a noisy inner valve on some cars) I'm going to get both the front top mounts and shock absorbers replaced this year.
Sadly, no hard and fast fix as each car seems slightly different with this problem.
I even wrote to Lexus head office to see if they could offer any advice, they replied but sadly could not offer any further help or suggestions beyond what I have already tried and proposed for the future.
I bought all my caliper parts from Lexus and it wasn't cheap (approx £300). On the plus side all my calipers have had a complete overhaul, on the downside it didn't cure the knocking noise! It's a really annoying problem and can spoil an otherwise fantastic car.
Once I get the mounts and struts replaced I'll post an update.