Many thanks Pepe and Steve for your swift replies.
I'm almost sure it's not the battery Pepe as I have tested the battery and charging system under load with a proprietary tester and both are reading ok. In answer to your question, the steering wheel moves quickly into position every time. The car starts without a problem about 75% of the time and turns over in a manner that would indicate a fully charged battery.
Today when I got in the car to leave work, the starter quickly jumped into life briefly before stopping. I turned the key immediately for a second time and the car started as it should.
Regarding the click Steve; The sound is same/similar to when the battery is flat or there is a bad earth (earth connection is sound).
The next time if fails to turnover I will pop the bonnet and get someone to turn the key whilst I listen for any clicks in the vicinity of the starter and let you know my findings.
Steve. I see you are in Derbyshire (I'm in Chesterfield). If the conclusion is a faulty starter, can you recommend a repairer who will do the job correctly for a reasonable price? When I had my GS 430 Sport, I used Lexus Sheffield and although they did a good job, they weren't exactly cheap.
Cheers
Paul.
My worst suspicions were correct. I booked the car into Lexus Sheffield and a week later the car was sorted.
1 x refurbished starter motor (£70) fitted at a cost of just over £600 including the vat. Gary the service manager collected the car from work and delivered it back valeted and with the usual complimentary tin of travel sweets. Fantastic service for which I cannot recommend them highly enough. Gary commented that they had never seen a problem with the inhibitor switch or a relay problem causing the fault. He also advised that due to the amount of work required to get to the starter, it was false economy to just replace the contacts because for the relatively small price difference, you have peace of mind and a 1 year warranty against mechanical and electrical failure of the starter. Unless of course you are carrying out the work for yourself and you have plenty of free time to carry out the arduous task of dismantling and rebuilding everything.