I omitted to say that after buying my RX300 I had an LPG conversion done by a VERY PROFICIENT AND CERTIFIED COMPANY - K Mugglestone of Boston, Lincs, that was 54000 miles ago and no problems have ever occurred.
After writing my problem I took the car round to my local excellent garage to have it checked by diagnostic meter, the fault showed as No 1 cylinder defective combustion, the fault shown was cleared and fuel changed to petrol, the car ran perfectly and even after a road test the engine check & vsc faults did not reappear.
I phoned Mugglestones and mentioned the problem and got the following information that would clarify where the fault lay, 'Start the engine on petrol, check that it runs ok, then switch to gas, still at tick over without load, if the engine runs normally there is unlikely to be a fault in the gas system at any speed or load BUT because gas is dry any fault in the ignition system will always show up on gas before it does so on petrol' , as the car has covered 69000 miles with original iridium plugs (they should be renewed at 60000 miles) the plug gaps will have widened considerably and likely to be the problem so change them,
'If the problem still exists it would most likely be down to a defective on-plug coil needing replacement.'
The car has been given to the garage today for plug change tomorrow - which entails removing the inlet manifold to gain access... ££££
Will let you know the result anon.
Ken