Thank you all for your replies.
I have clarified the shock absorber issue with the Lexus dealer and their engineer.
Last year during the road test on the 'full' service, they heard a 'knocking' sound although on visual inspection, it looked good. They therefore advised that I had it changed last year which I refused (thankfully).
When I sent it in for the interim service last week, the same engineer who did it last year said it looked completely fine and therefore doesn't need replacing.
They apologised for asking me to replace it last year and we shall await the full service next year before I make any rash decisions so for now I will be keeping the car.
I will find out about the spark plugs as well.